Bathroom Remodel and Old Plumbing Question?

[1Newaygo]

I am starting to remodel my bathroom and opened up the wall with the old plumbing (galvanized) and the old Drain pipe has a separate Drain Pipe for the Bathroom Sink running parallel to the Drain Pipe for the Kitchen Sink and it runs across the wall which has a true 2x4 with a 1 3/4 x 1 5/8 attached to the 2x4 for these 2 drain pipes to fit inside the wall. Is this the way in the Old Days they would run the drain pipes. As Today they just connect all the drains in the same usually PVC Pipe. Not seen drains running side by side through a wall have not got behind the shower area yet as how they might connect but maybe to the Vent Pipe that goes up through the roof. As the Shower is the only 1 in my house so tat will be the last part of the walls I remove. Just curious why the drains were not all in the same run / pipe through wall studs maybe why the wall is deeper to accommodate both drain pipes?

Looking to replace all the supply lines and waste drain pipes once the walls are wide open, with all the walls, tile and tub removed.

1Newaygo

 

[WorthFlorida]

A picture would help but what are the pipe diameters and where is the vent(s).

 

[Sponsor]

 

[1Newaygo]

See Attached Pic as for the Vent Pipe it is a Cast Iron that drops thru the Wall behind the Shower Wall Section I have Not Removed Yet. Sorry it is not a great pic.
1Newaygo

 

[1Newaygo]

Here is another Pic the Bathroom Sink Drain Pipe OD is 2 inch yet the Kitchen Drain Pipe OD is 1 1/2. Yet the Bathroom Sink Drain Pipe Size was 1 1/4 and the Kitchen Sink Drain Pipe was 1 1/2. I would have thought the Kitchen would of had the larger size Pipe and the Bathroom the smaller. Learning something each time I work on my 1940's 2 Story Bungalow House.
1Newaygo