Generally, due to the fact that vintage 50 Fathom Blancpains gained remarkable interest in the market (partly attributed to the recent homages to the original ones presented by Blancpain in the recent years) and the rather common technology used, there are some caveats in buying vintage ones (e.g. avoid AS2063 movements), and I strongly suggest to have the watch inspected by a specialist (or by Blancpain).
If you have further questions, please do not hesitate to come back!