There’s no mystery to that. Before modern bioengineered insulin the major type available was porcine insulin. It’s very similar to human insulin. It must be injected like any other insulin (as a simple protein it's broken down quickly in the digestive tract).

https://go.drugbank.com/drugs/DB00071#identification-header

Now and then it comes up and diabetes discussion groups, what would we do in the event of a widespread disaster without access to the pharmaceutical system? Pig pancreas. From what I recall it requires a very large amount of pancreas, perhaps several organs per dose, and a centrifuge.