With most Military Contractor MFGed Guns (SACO/Lowell/Maremont/New England/etc.) generally the serial number is on the trunnion from what I have come across.
The next largest group of guns out there are the RIA guns. The majority of these the Serial # is on the sheet metal, but as referenced by the thread from FALDOC above, some earlier RIA guns were made with trunnions from demilled ex-mil guns so you could see a serial on the trunnion and/or sheet metal. The later generation RIA guns with their own mfged trunions are just marked on the sheet metal (both maker and serial) and the trunion is clean of any markings what-so-ever.
I am sure there are aftermarket guns made by various C2s out there that have marked some combination of the trunnion, channel, or left/right rails. (never heard of the rear bridge being marked before but I guess it is possible)