This forum is really great with lots of knowledge and information. I think we can assume that this type of frame was built until 1938. Except for the M24 which had no sidecar connections and frames made of Reynolds tubing.
It's amazing what details come to light.
This type of frame was even used in 1939. See below photos of a Dutch Army KM21, which was delivered early in 1939. The rear frame is original, it still has the original Dutch Army paint on it (same as for the front part of the frame).