Don't think mine (mid 60s as I mentioned in an earlier post) has a number so "3500" is more likely to signify that its a later one. Although good for their day, they were nothing more than mass produced instruments and I doubt it was worthwhile them adding serial numbers.

Incidentally, I spotted several new ones hanging in a shop last week - all now made in China (where else!) Same name, totally different construction. One of them had the only backward bow I've ever seen on a new guitar - and the shop didn't appear to have noticed! The moral, therefore, is, check before you buy and, if you don't know what backward bow/reverse relief is, take someone with you who does.