Good morning
Very smart - and I see from the sticker that the frame is a pretty decent spec as well. 501 must have been a big seller for Reynolds, you see loads of mid-quality classic steel machines out there built from it, and to a lesser degree plain-gauge 500.
I had one, with all the gold bits and it was a pleasant ride although not particularly light. I bought it because it had the Reynolds sticker and at the time I couldn't afford a 531 framed model and I suspect that many others did the same.
I also suspect that 501 was part of the reason that aluminium frames started appearing in much greater volumes.
My 501 frame failed within a few thousand miles, the down tube cracked at the lug with the head tube allowing you to pull the head tube away from the down tube. I did get a replacement frame but that was nicked before it had done many miles.
Whilst in theory 501 was available as "Tubes, forks and stays" the only bikes I have ever seen were main tubes only and 501 main tubes only isn't special enough to justify craftsman who spend hours over each joint building the frames and too hard to build using robots.
For the high volume makers dropping thin steels and investing in automated aluminium welding would seem to be the future back then.
Bye
Ian