I’ll throw this Technotrat frame into the mix as well that I recently bought in Australia. Again, similar seatstays with heart on the headset lugs only. Mine is stamped with the size on the BB but also has EC stamped faintly in the seat tube indicating “Europa Cycles”.
The other difference that can connect these frames are the dropouts. Mine has Gipiemme rear dropouts with unmarked drop outs on the rear. I’ve seen others with Technotrat dropouts.