Frame Weight

Assuming you mean Reynolds 531c? At a guess, the 531c because at least the three main frame tubes would be butted whereas 500 - chro-mo is typically plain gauge. But there were lots of 531c frames made with plain-gauge chro-mo chainstays & seatstays so it could be very marginal.
 
The 531c frame will be lighter because something would have broken and fallen off !

Just joking :)

Shaun
 
normally 531c tips over 2kg , std 531 are normally around the 3kg mark .

the lightest i have seen is 531 sl at 1.8 kg ish .

normal gas pipe is anywhere from 4 kg upwards .
 
Montydog. Did you mean the opposite.? The butting makes things lighter. Grammatical error?
500 is pretty hefty.
 
Given the headset and bottom bracket are frequently heavier there will likely be around a 0.5kg difference, but obviously there will be variation. 500 is actually OK for a runabout.
 
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