The factory YZ623 ccm twin cylinder racer developed from the 1969 AS1 based model up to the latest 0W15 twin in 1975.
The 2 pictures from the early 1969-1970 version show clearly the combination of standard parts and factory specials, like the wheels/hubs. (You can "click" picture 2#-3#-4#).
On this picture it seems that even the standard AS1 rear shockabsorbers are fitted!
The "C" model from 1972 has already improved a lot, with the new bore/stroke of 44x41 mm instead of the old standard bike 43x43 layout. The gearbox is now 6-speed, Watercooling is added, a dry clutch, VM28SC carburetors and a double twin leading shoe front brake made it a very good bike.
Here you can watch Ferry Brouwer working on the Chas Mortimer bike during the Swedish G.P. of 1972. It looks like it was a scorching hot day and paddock conditions were very, very different from today's luxury.......!
The frontbrake as fitted to the YZ623 here is the same as later on the TA125, a reversed DS6 brake drum with a CS3/CS5 right hand brake plate. This brake was exchanged for a proper one on this Dutch bike by Ferry Brouwer the week after this picture was taken (see link with early Dutch Team)