Reynolds 531 frame single speed
No. 531 is a frame material from which any type of bike can be built. It's actually Reynolds 531 a brand name, from which different tubesets are made and they have also sold small quantities of raw tubing as well as custom butted tubes. That exact tubing is now somewhat dated, they sell newer compositions with different numbers now.

So all you can really say about a bike with 531 tubing is that it's probably more than 5 years old. It might be a touring bike, it might even be a load bike, but more likely it's a road or track racing bike.

If you're looking at buying a frame, first make sure it's designed to be built up the way you want (most importantly for you does it have horizontal dropouts). Then measure it and make sure it will fit. Best done by finding a bike that does fit you and comparing the measurements, since building up the frame will cost money.

Also, make sure you have a fork for it. Most likely a 1" threaded head tube, 622mm or 630mm fork. If it doesn't come with one it's very important that the first thing you acquire is a suitable fork. In the event you can't get one you would need to get one custom built and that will be expensive (this will most likely be because the forks you can find that fit look ugly because they're cheap). Best to make sure you can get that custom fork if you need it, before even buying the frame.