Thank you to all who have commented.
The manual that Willem was kind enough to provide is indeed for the Benz Bz.III motor.
A further search of the Krakow Muzeum web site and I have found similar manuals for the some of the Sechszylinder Benz Flugmotoren including the Bz.IV and what is referred to as the 200Ps ( hp ) engine and Sechszylinder Benz Flugmotoren type FB type FD and type FF
Benz used internal type ID's for some ( most or all ? ) of their motor as follows:
manufacturer's designation: Type FD Bz.?? 85-95 PS
manufacturer's designation: Type FD Bz.II 100-110 PS
https://en.wikipedia.org/wiki/Benz_Bz.IImanufacturer's designation: Type FF Bz.III 150 PS
https://en.wikipedia.org/wiki/Benz_Bz.IVAll of these manuals provide several nice drawings of the respective motor. However, the drawings in the 150Ps manual and the FB FD FF manual are very similar. It can be assumed that the drawings in the 150Ps manual are for the type FF ( Bz.III ) motor but it is not clear which motor the drawings in the FB FD FF manual represent - they are very similar to the drawings in the 150Ps manual but have small differences, perhaps the type FD or early FF.
I have extracted these drawings from the 150Ps and FB FD FF manuals and made a few notes which I can provide in a .zip file if anyone is interested.
Unfortunately I have been unable to find similar images for the Bz.IIIa which is the variant I most interested in. The Bz.III and Bz.IIIa are only similar in that they were the same power. The Bz.III has it's intake and exhaust ports on the same ( left or port if you prefer ) side of the cylinders whereas the Bz.IIIa has it's intakes on the left ( or port if you prefer ) and exhaust on the right ( or starboard if you prefer ).
German motor designations follow a pretty simple pattern:
up to 80 PS x.0
80-100 PS x.I
100-120 PS x.II
120-150 PS x.III
150-250 PS x.IV
250-500 PS x.V
where the x is something related to the manufacturer i.e Bz for Benz or D ( I think ) for Mercedes
My interest in the Bz.IIIa is for a recent project of Hansa Brandenburg W.29. My current thinking is to build it like serial 2670 which was the one taken back to England, repainted ( apparently ), and marked with British roundels. The serial number indicates that the aeroplane was powered with a Bz.IIIa ( Windsock datafile 055 Brandenburg W.29 by Peter Grosz page 32 ).
Pictures show that the exhaust manifold is clearly on the right side of the engine so it is clearly not Bz.III powered.
However, the radiator style is the same as on later W.33 Mercedes D.IIIa and D.IVa (page 13 picture 32 comments).
So, is it possible that 2670 was powered by Bz.IIIa or maybe D.IIIa or D.IVa ?
I have enlarged the few photo's I can find for 2670 that show any kind of upper engine detail and it really isn't clear but most likely not D.IVa.
My dilemma is that my kit of choice is an old 1/72 Meikraft W.29 limited run low pressure injection molded kit and the provided engine is little more than a lump of plastic and it will need to be replaced with something that looks reasonably correct.
What to do , what to do
cheers, Graham