If you peruse the previous 15 pages of this "Military Shotguns" forums, you will find numerous threads on 1200 Trench guns, including many photos.
The 1200 Trench Gun was equipped with a proprietory Heat Shield/Bayonet Lug assembly.
Winchester 97 or 12 Heat Shields, or Ithaca 37 Heat Shields would look similar, but are not legitimate for a 1200 Trench. It all depends on how accurate you want to be on your clone.
Monitor eBay and GunBroker websites - original heat shields, stocks, forends, and even barrels, show up every now and then.
You will also have to think about re-anodizing the receiver to a matte black finish, and parkerizing the Bolt and Shell Carrier to a matte black finish. Originals were a "greenish/grey", but difficult now to reproduce as t's my understanding that some of the chemicals used are banned as environmentally unsafe.
Below is a 1200 Trench Gun clone I was able to put together with parts I sourced from the above noted websites. However, took about 16 months to find everything.
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