It turns out they were using the BS as an indicator of basic intelligence and willingness to work and achieve a goal. And that’s what’s going on here with regard to sheepskins: the employers are using it as an intelligence or psychological test. They don’t care about the last course or really any course.

PS: Parchment is animal skin prepared for use in writing; vellum is specifically lamb skin, which provides a superior surface to parchment. Both produce very durable writing surfaces, much more durable than paper or modern electromagnetic and optical media, but not as good as burnt clay.