My Sun wasp of 1950/51 has the marke on the nearside captains seat lug. RT316. I believe R is the date letter. T for tandem and 316 the serial number. The giveaway for mine is the forr pedal lug on the drainpipe.
Here's a pic from the Sun 1938 catalogue I dredged up some years ago. Can't make out whether the rear drop outs are rearward facing-not on mine.