Need help - Reynolds 20-30 or 531?

I have a couple of Raleighs from the 70's. One is a "Record Ace" and I
know that the tubing is Reynolds 20-30. The other is an unknown
Raleigh, because all of the decals are gone, except the "Made in
England" one.

So how do I tell if my mystery Raleigh is Reynolds 531 or 20-30? It
seems pretty light, but all I have have is the serial number. The
original paint - sky blue - could have been from a number of different
models. Are there any tell-tale signs? Thanks for the help.