Advise needed please!! Rear brake cylinders Mk1 Convertible

Hi there, having owned a Mk1 golf since I first passed my driving test would you believe I am finally joining this club!! We are currently lovingly restoring a Mk1 Golf Convertible 1.8 1988 and it needs some rear brake cylinders. Unfortunately, we've come to realise that it's not as easy as popping down to the local parts supplier! There are apparently 2 types of sizes - 14.29 or 17.46, those we have taken off are 14.29 but when purchased the ends appear smaller?? We were thinking this may just be because they are not original parts but don't want to put the wrong ones on! I hope this all makes sense, I'm hoping there is someone out there that has experienced something similar and can offer some words of wisdom as it's driving us mad!! Thanks