My answer may come quite late, but if helps, the reason why the medal model used between 1854 and 1880 is so scarce rest in 2 factors, 1) the “new model” is commonly believed that was introduced in 1880 but there’s evidence that is older (around 1875) but only legally recognized in 1880 so your diploma must have come accompanied by new model medal. 2) the old model concession was very scarce, and the majority were given in the 50s so for the reintroduction period (1876-1880) there were not many around and for the surviving ones there was an "invitation" to trade them for the new model (the original model and quality was so unappealing that there was no incentive in keeping it, while the new one was very attractive back then). Once turn in, my guess is that the government eventually just melt them all… unfortunately I don’t know any surviving piece in any private or museum collection including the Tenreiro collection from where that imagen was taken. BTW wonderful example of Diploma… congrats!
This is the Diploma for the first type medal