[Reviving this thread just to provide additional info plus physical confirmation of parts compatibility.]
I just spoke to Fernco on the assumption that they would be able to provide confirmation about to whether or not their SD-200 drain seal is a direct replacement for the Mustee 42-318. Frustratingly, the Fernco person I spoke with informed me that they had no ability to cross reference another manufacturer's part number with their part. I found this utterly bizarre (and highly frustrating).
I wanted confirmation because although
on paper the Fernco part closely matches the actual measurements I took using a digital caliper of a worn out and leaky Mustee 42-318, I wanted official confirmation that Fernco produces the SD-200 seal expressly for the Mustee 42-317 drain assembly. I learned long ago the benefit that often results from questioning something in print.
I then called Mustee to ask what the difference is between their 42-317 drain assembly (the model number embossed on the drain assembly I removed the seal from) and the current version of that "same" drain assembly, denoted "42-317A". I assumed that the "A" appended to the end of the new part number reflected some sort of change or update made to the original version. Well, the person I spoke with put me on hold to check with someone else but when they returned they stated that there is zero difference between these two assemblies. Based on this, my guess is that they went through the trouble at some point of changing, but only slightly, the nomenclature they utilize to keep track of their various offerings. IOW they didn't do anything to otherwise change the physical design of the assembly; they just changed the part number they use to denote it.
Over the weekend I encountered the unexpected need to source a replacement drain seal for a Mustee 42-317 drain assembly. However, no one open on the weekend in my immediate area had any stock of the OEM seal. It then occurred to me that I could maybe purchase an entire drain assembly (my local HD did have a few of the 42-317A drain assembly in stock for ~US$20) and simply cannibalize the seal from that kit (I was able to confirm myself that the drain seal that comes with the 42.317A is in fact the 42-318 - an internet search bore that out). In the end, however, I decided to wait and on Monday (today) called my local plumbing supply house. Turns out they had the Fernco SD-200 in stock. So, I visited their shop and brought with the worn out Mustee seal so I could physically compare it to the Fernco.
As you can hopefully surmise from the following photos, I can now confirm that the Fernco (I purchased mine from my local plumbing supply house and its grey in color though I've read elsewhere that sometimes they can be black in color) is, for all practical purposes, identical to the Mustee:
Aside from the color difference, the Fernco features symmetrical interior and exterior "ribbing", which is in slight contrast to the Mustee version which has a symmetrical ribbing pattern on the exterior but an asymmetrical ribbing pattern on the interior (compare the two parts and you'll see what I mean), necessitating, as per Mustee's installation instructions, that there's needs to be installed with the embossing facing
up (the one I encountered was installed upside down, go figure).
Meanwhile, though Fernco provides zero guidance regarding installation of their version, I suspect, given it's reliance on a completely symmetrical ribbing pattern, that the orientation of there's doesn't matter (with theirs there's no obvious "right-side-up"). Also, the Fernco, likely because it's new and supple (as opposed to old and dried out) is ever so slightly
thicker (we're talking thousandths of an inch here folks) - which is a good thing given that the likely reason the Mustee periodically leaked (hence, my reason for wanting to replace it) is not because it was installed upside down (I doubt this made a real difference) but because it had shrunk from age and was no longer making a watertight seal between the tapered ID of the existing drain "seat" and the OD of the original seal and/or between the ID of the original seal and the OD of the existing 2" Sch 40 PVC shower riser.
That said, having since installed the Fernco seal in the existing Mustee drain seat, it's obvious that Fernco produces this seal as a direct replacement for the Mustee seal, even though nether party will divulge and/or otherwise admit to this.
As an aside, I sought confirmation from Fernco because I initially assumed that I'd have to order their seal (I couldn't know for sure over the weekend if anyone locally stocked their seal and I was getting ready to place an order from one of the online supply houses and figured I'd include the Fernco seal in with that larger order) but, when I found one locally, I simply purchased it, even though the local price was almost double that of the online supplier. I didn't want to wait even a day to be able to complete the associated job.
Anyways, hopefully this helps clarify the earlier portion of the thread (which I found to be a little cryptic) and also removes any doubt anyone else might have about whether they can avoid paying Mustee's highway robbery price of US$50 (including shipping) for their seal and instead go with the far cheaper Fernco version.
Finally, be careful when going the Fernco route.. Fernco produces two shower drain seals (which look identical until one compares the measurements of each model that Fernco provides on its website) and the one that both Fernco and retailers list as being the one compatible with Mustee shower drains (the SD-210) is
not the version I needed. The SD-210 is much bigger is every dimension. So, while the SD-210 may fit some other model of Mustee shower drain assembly, it most certainly will not fit the 42-317 or 42-317A (again, you can disregard the different part numbers - they are in fact identical drain assemblies). This is confusing (especially since the product description for the SD-200 is silent on the matter of compatibility) and produces an unnecessary mystery. In short, if you encounter the need to replace the seal on a 42-317 or 42-317A then the Fernco SD-200 is the seal you want.
