Bob,
You are entirely correct. I've built several pistol locks from castings by Blackley. The castings are very good but the small internal parts are a nightmare because, cast from original locks in their finished state, they have no extra metal left anywhere. Just cleaning them up can ruin them. Now I buy the frizzens, plates, and flintcocks and then make all the internals from scratch. That works the best for me.
dave