Those locks you mention were only on very early Mk4's and were used for probably about a year. The pegs were prone to snap off the key aswell, more than likely changed for obviouos reasons.

If you look on ebay theres some aftermarket locks that a company produced to replace the diamond locks because they weren't very secure

I've not tried fitting an earlier lock myself, but i know if you go to Ford and manage to get a Mk3 door its actually a mk4 door with an adaptor plate so you can fit the mk3 interior door handle, and everything else is a direct fit