We keep thinking Allegro's serial numbers are sequential and logical. I am not so sure. Looking at the speedbicycles bikes, we find -

No. 7 Tour de Suisse dated to 1936 with serial number 6347 on the left rear dropout (behind the axle) and fork crown

No. 7 Tour de Suisse dated 1937 with serial number 16464 on the left rear dropout (between the seat and chainstay junctures)

No. 7C Weltmeister, also dated 1937, with serial number 101370 on the left side seat tube - unfortunately, we don't get a photograph to confirm it is the now familiar lozenge on the side of the seattube.

Now there is also a track bike labeled "Allegro Piste Special" with NO markings except for the lozenge serial number 225909 - someone who knows their track parts could probably better determine the date on this bike. I suspect it is later than 1950 ....