Thank you,
MauriceMoss , because this one was driving me NUTS. The non-Fischer sand-cast BB and the pinned/peened/dimpled chainstay sockets, coupled with the serial number location and the serial number itself all pointed away from Allegro.
From what I gathered during my Allegro sleuthing phase was that they standardized on the seat tube lozenge before WWII - if the dates given for certain bikes are reasonably accurate.
This one, for instance, allegedly has the lozenge.
If you're really bored, or have info you can add, I started assembling an Allegro database a while back ...