Hi, I'd guess you're having a laugh, however on the off chance you're not.
531 is a type of reynolds tubing, it comes in various types, 531ST (touring) 531P (Professional) etc, etc.
500, not sure what this is, however there is a type of Reynolds tubing called 501.
500 is just plain-gauge (i.e. no butting at the ends & uniform inside diameter throughout) 501. Both are seamed (i.e. rolled from plate and welded at the join) rather than seamless (drawn from solid steel billet) tubing, using a chrome-molybdenum alloy steel. As against manganese-molybdenum steel used in 531 tubes (chrome-molybdenum takes better to the use of welded rather than brazed frame building methods).
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