Nothing has come to light yet to suggest that the S1 and S2 used different frames. Since they both had the same stroke and shared a common (dimensionally) bottom end, there is no technical reason that the 733cc (6hp) S2 would require a larger or different frame. There is evidence in the 1921-23 spare lists that the 3-1/2hp Sports frame did go through more than one iteration; part number 720s being assigned specifically to 1921, and 1180s to 1922-23. I do not know the details as to what was different. While it is possible Douglas allotted it a unique prefix to denote a frame allocated to 733cc verses a 494cc, no unused prefixes are available in the range (AF through TF), but more on this anon
Things get a little more confusing regarding the CF prefix. CF seems to be assigned to the all-chain drive CW side-valve model, judging by the number of examples in existence. So the reference to a CF prefix being assigned to a "3-1/2hp OHV brazed frame" in the appendices of Jeff Clew's book is at first glance apparently an error. There are several other typos and errors in those appendices that have subsequently come to light through research that make it a possibility. However... at least four people that have OHV Sports models claim to have CF prefix frames. This seems to be too many to be a case where the information was taken down incorrectly, or a typo in the LDMCC registry. It looks like Douglas used CF on both the CW model, and the OHV Sports models.
Just to muddy the waters further, there was a BF frame prefix according to Clew, no examples known, assigned to simply a "2-3/4hp". So was this the frame prefix that should have been assigned to the CW side-valve model? Well not so fast, because in the 1921-22 OHV Sport spares list, there is mention of pistons for a 2-3/4hp OHV Sports version! I can not recall ever seeing this cataloged properly, though there is a mention - almost as an afterthought - of in in a 1921 3-1/2hp Sports model brochure. It is possible the frame prefix BF belonged to the 2-3/4hp OHV Sports model. Though as with the 6hp verses the 3-1/2hp, I do not know why a different frame would have been required (and none is given in the spares list.) I have not seen a picture of what a 2-3/4hp OHV Sports model looked like. The reason for this tangent conjecture is the engine/frame/gearbox prefix allocations at the time followed a pattern, which quickly fell apart for practicality reasons shortly after the introduction of prefix codes. So we have:
Model, engine prefix/frame prefix/gearbox prefix
3-1/2hp OHV Sports, AE/AF/AG
2-3/4hp OHV Sports, BE/??/BG
6hp OHV Sports, CE/??/DG
A,B,C and E for engine, F for Frame, and G for gearbox, a real cypher!
So we see frame BF fits nicely into the pattern for the 2-32/4hp OHV model, which chronologically would seem to be the second model offered. So by rights the 6hp should
have frame prefix code CF, but we have seen this has already been used on 2-3/4hp side-valve CW models too. And the pattern is already beginning to unravel as gearbox prefix CG was already allocated for something else.
So my best guess is frame prefix AF was for the 1921 Sports model (Jeff Clew states it was for 1921 only) at which time only the 3-1/2hp was offered. Then came the 2-3/4hp with frame BF, though whether any were ever sold is a matter of speculation. Then came CF, which was needed to denote the changes between the 1921 3-1/2hp frame, and the 1922-23 version. This prefix was also allocated to the 2-3/4hp side-valve model (or vice-verse
), for reasons unknown. Alternately it is possible the AF/CF denote the differences between the 3-1/2hp and the 6hp, but this then means the AF entry in Clew's book is in error too.
Further confusion is the 1921-23 spares list states lists one frame for 1921 models, and another for the 1922-23 years (as noted above.) But the 1922 spares list (issued May 1922) only mentions the earlier frame! If that were not odd enough, the earlier spares list mentions the 2-3/4hp pistons, but no cylinders or heads to go with it! The later spares list, issued Dec 1924, does the same and adds the interesting tidbit certain parts (obsoleted) were available for the 1920
Anyway, if anyone can throw more light on the subject I am all ears.
1921 3-1/2hp OHV Sports Brochure
1922 3-1/2hp OHV Sports Handbook
1921-1922 3-1/2hp OHV Sports Spares List (illustrated)
1921-1923 3-1/2hp OHV Sports Spares List (unillustrated)