Not familiar with enamel proofs for 1977 ( or other years) so look forward to someone else knowing and sharing something about them.
I am aware of some ceramic or porcelain cards and sets Topps experimented with, I think in the 90s. That included 'reprints" of all the Mantle cards in porcelain.
There are some very sought after 1977 Topps proof cards listed in the SCD Standard Catalog that involve players in different poses or on different teams than their issued card that year, the best known being the Reggie Jackson as an Oriole. All of the SCD listed proofs go for big dollars these days.
Here is an example of a 77 unissued proof
