Help Identifying Frame (S31 / R17)

[Ats666]

Dear BSA owners,

I'm Looking for info regarding bike frame / parts on below photos that has been recovered from an old barn. Engine (#Y7 1131) has been identified By BSAOC dating officer and seems to belong to 1931 Model S31. However as the frame number was misspelled this could not be identified at the time. There is #D6 3074 on the frame (Under saddle). With reference to BSA frame and engine numbers table this would correspond to R17 model from 1936. However, having twin down tubes, that seems bit odd. Afraid this bike has been assembled from two different frames. Perhaps partially from S31. Front end has been fitted from quite a bit newer bike at some point (Pannonia), yet tank seems from older model BSA as it is without panel etc. 
On one down tube near the engine, there is also #RD7 2423 4 stamped on it. Believe that to be part number, as same appears on another BSA frame.
Would very much appreciate all information that can help to identify frame and other parts that are making up this bike.

Best regards,
Andrus

[AWJDThumper]

I'm no expert on BSA's of this period but presumably this is a 1931 S21 engine in a 1936 R17 frame (except front-end). Your frame seems to display the threaded holes either side of the headstock which were used for the front tank mounting bolts on 1934 and later BSA's. Presumably, to fit the S21 engine, the front and rear down tubes were chosen to suit this engine and were not those from an R17?

[JulianS]

D6 prefix frame numbers were found on a number of the 1936 models;

3.48HP R17
3.48HP R20 Blue Star
4.98HP Q21 Blue Star
4.98HP Q7 OHV
4.99HP W6 side valve

Second photo is the R17 from 1936 brochure

[Ats666]

Thank you for replies. Much appreciated. For sure there is mix of frames here. Will try to get more info regrading frame number from archives to see which model it actually belongs to. 

[Hoppe30]

Andrus,

I may have the correct front frame for that bike. Send me a PM if you are interested.

Kind regards,
Leon