I am looking at a home built in the 40's with a 2" pipe with elbow approximately 1" above the basement floor -it is capped - the house has been connected to public water and sewer since the mid 70's any idea what this pipe could be it also has a relief valve at top of elbow (I think that's what it is).

Appreciate your thoughts?

Post a picture.

Your description doesn't make much sense, I was OK with a 2" capped pipe. That could have been a shower stub up. But you muddied the waters with a elbo you say was capped. Was there a short piece with a cap coming out of the elbo? What material are they. Galvanized or cast iron?
Then you really confused me with,
it also has a relief valve at top of elbow
1- Drainage systems don't use relief valves
2- Unless this is a special elbo with a heel inlet that's impossible. I started in the 40s so I know what was used in building homes back thern. We REALLY need a picture, Back to you, n Tom