Hi Roger and Leon,
Yes Douglas did certainly mix and match parts, usually i think to use up old stock and get the bikes out the door, but during WW1 and later owners mixed and matched everything over and over as well to keep the bikes on the road!
I am no expert on this subject either but when I started my 1915 project i didn't have any rear brake parts at all and was originally going to use footboards and pedal 109D on the front (detachable) Tube (609D). I found a Detachable Foot Brake Mounting 615D with the small brake pedal before i found a 109D pedal, so I then investigated how this might go with footboards. I then realised, after looking at the 1915 preliminary list, the 1916 handbook of the Douglas (I have not found a 1915 one...) and pictures of WW1 dispatch rider bikes that the detachable brake pedal mounting may not have been intended to go with the footboards by Douglas.
After the decision to go with footrests was made, I purchased some footrest rubbers from the LDMCC.
The footrest rubbers I got from the club have "Douglas" on them and are riveted together with the end plates, the ones in the 1916 The Handbook of the Douglas look very similar (612D) but are shown with threaded rods and nuts to hold the end plates on. Interestingly the 1916 Book lists a part 608D Rivets for footrest pad, so maybe after the bolted ones fell apart in action they decided to rivet them together!...I will go with the riveted version from the club!
Note that the 1915 preliminary list was probably released in late 1914 and the detachable rear brake mount 615D may not have been introduced by then and hence it looks like it escaped theescaped the artist's impressions (if you look closely at the pictures), but Douglas do make a statement "New foot Brake" under improvements for 1915.
So the 1915 Preliminary List shows that model U and W have footboards and a 109D pedal at the front and model V has the rear footbrake pedal mounted on the rear footrest tube. the ladies model X has the same setup as a model W from the pictures I have seen but it is not shown clearly in the list.