It's a budget bike by Dawes standards, but it isn't made of gas pipe. Reynolds 500 was simply the plain gauge version of 501, with identical metallurgy. That only appeared a few years after 501 did in 1983, so I would date it to the second half of the 1980's on that basis. I picked up a Dawes donor bike with a 500 frame beore the virus, and it is only maybe half a pound heavier than a butted frame. The forks are fairly light by steel standards and they aren't built like a tank. The mystery bike is probably 72/72 geometry and would ride pretty much like any lugged Galaxy I'd expect, but cost less to buy.
