It is now a documented, 20 gauge R10. The crossed lightning bolts mean it was proofed in a finished state, common for that model. There is enough choke in the left barrel that the factory didn't want you using pumpkin ball slugs in it, and told you so. The other barrel is more open. 16.0 is a big 20 gauge bore, and many Darne 20s like this one are running around with a .630 bore, effectively backbored before it was a shotgunning buzzword. The gun is for sure post war, the second big war.
The SPC prefix serial number keeps us from giving an accurate date of manufacture, but, figure 40s-60s and you are in the ball park.
I've got a very similar 20 gauge:
SPC prefix serial number also, along with 16.0 bore.
Best,
Ted
