The tube holder makes perfect sense, Feltwad. Nice "tubes". This type of ignition would be much more direct and likely more sure than the later percussion caps.
Tube locks were predecessors to cap locks, I assume.
A detonating compound for tube lock tubes:
5 parts potassium chlorate
1 park sulfur
1 part charcoal
was used in tube loads, but is apparently a dangerous compound to use, handle or make.
I made some way back in the 70's for my home made tap-o-cap percussion caps when I could
not buy percussion caps in Smithers, B.C. This compound worked much better than kid's toy
caps, recommended for these percussion caps.
This compound, like Pyrodex, due to the use of Chlorates in this case and Perchlorates in Pyrodex, makes for a VERY corrosive fouling from the compound.