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Here's a new 5 speed.
They offer a 14-24 which your mech might be able to cope with. The TdF doesn't really like anything over 22 teeth, but I think your JUY 51, because of its different jockey cage, would probably manage 24.
Nearly all the screw on blocks offered now have at least 28 tooth bottom - which seems to assume the purchasers are are bunch of wimps!
I can't check my own stock atm, but I may well have a spare 14-18. Sadly, I'm unlikely to be able now to cope with an 18 bottom anywhere apart from Regent's Park.
It looks fine but unfortunately it has made no difference at all, the chain is still not engaging with the freewheel properly and to top it off while I was out on a test run earlier the freewheel packed up completely and I had no drive at all.
So I had to walk home albeit not far.
I'm now on the hunt for a freewheel, 5 speed.
I'm going to wait for something decent though as I've been stung before buying second hand freewheels that look great on the outside but totally knackered within.
@clubman it would be great to hear about these bikes being raced back in the day, perhaps a couple of photos too.