The gearbox on the grey flash( bike 2899) is an Albion (I think) but perhaps David D can tell us what type?
there are lots of Vincent pictures here
I believe this to be F5AB/2B/2921, which was considered to be the Works Flash. The above would have been the published photo from the original that has been used in most of the Vincent literature. The gear box is one of the 6 Burman BAR boxes made at the request of Phil Vincent around 1934? It was used for the 1935 Series A TT racers and special machines throughout the years. It was said to be an all roller bearing box and quite nice in operation. This is the box that was infamously fitted to the Grey Flash TT racers in 1950. It unsurprisingly was not quite up to the task and the staff at the TT had to convert the racers to the Albion boxes that had already been fitted to the production Grey Flashes many months earlier.
The photo is probably how the bike looked like in 1949 as one of the Grey Flash prototypes, the other being the Eppynt Racer. Note that there is no F106/1, but merely two tubes. The foot rest plates are quite small and there are no rear set controls for the shifter. It is a plain case with a Vincent transfer on the tank. The front upper spring tube has a hole drilled in the top to release air pressure trapped inside the spring boxes, which affects the damping rate.
There are lots of black parts on this bike including the RFM, which did not match the UFM by number. You can also see the shiny chrome Lightning carb adaptor that came on most production Grey Flashes.
Most interesting was the suspension. Of course, the fully sprung seat, which is the best, but if you look at the position of the front lower link it is surprising that a bike this light maintains such a low position for the eccentrics. The springs were clearly designed for maximum handling and braking and were not stock.
David
