I can shed some light on this subject....I am/was a Propmaster & Production Designer as a vocation for 45 years and still counting ...
Stembridge Gun Rentals (Hollywood ) did have trapdoors mocked up to look like flintlocks...
As did Disney Studios & Western Costume Rentals...
I have had them on shoots .... I suspect , however these were not the case on the recent "Making of America" series.
Most if not all are pretty sad condition ... Being many were produced 7o - 8o years ago.
Stenbridge collection is mere shadow of it hay day years, most of its was inventory was auctioned & sold off some years ago.
What I believe you may have seen were, Electric fired guns.... These have small 9v battery and fire a squib.
I saw the Dan'l Boone & Tecumseh episodes , and a bit of the Lewis and Clark and Andy Jackson , I found them lacking .