Hi Roger,
Thanks for the update/correction.
Reliability? David writes that it broke a crankshaft!
I have to think that each of those long 'pushrod' tubes contained, in fact, the central (rotary/oscillating) part of a special, much-extended rocker.
No great puzzle about how the outboard ends worked. The inboard ends could each have been activated by a face cam on a common shaft.
I wonder why Bristol decided to turn their engine upside down.

I found that Milntown does not have 1923 Autocar vols so could not investigate further.
Of no relevance at all but a great find, in an earlier volume this delightful Edwardian advertising card seems to have been left as a book mark for over a hundred years.