Need help identifying current pressure tank for equal replacement!

Hi,

I need someone to help me figure out the size of my current pressure tank. I'd like to go with a smaller pressure tank, but I don't have a cycle stop valve installed and because I have a TEE directly off of the pitless going another way from the pressure tank I'd have to dig up around the casing to install it, unfortunately I just got done closing it back up!

My current pressure tank:

Bekeley
Model No: S40519
B/M No: BT220
Code No: 1K90S (maybe 5, instead of S)

I'm not sure the size of this unit, it's fairly large... I had no problems with pressure though I have it tee'd off 100' to my house and 100' the other way to the pressure tank in the pump house. There's a diagram of my setup in this forum as it used to be a 2 house setup (now it's only 1) that many of you helped me with and it worked out perfectly!! I just forgot the CSV and will add it down the road but for now I have to find a replacement PT..

Thank you!