Mike,
Nothing like what you have is described in Canfield's Complete Guide To United States Military Combat Shotguns. Serial number ranges for the Vietnam era six-digit serials are listed in the high 900,000s though he does indicate that the full range of serials is not known. And there are no riot guns listed in that serial number range, only trench guns. A trench gun without the handguard/bayonet mount should still have the screw notches on the underside of the barrel. Even so, the forward sling swivel for trench guns was on the bayonet mount, not the barrel stud.
All the Vietnam era Ithacas illustrated are marked U. S. on the right side of the receiver up front, closer to the bottom of the barrel than to the loading/ejection port. They have the Ithaca anvil just below that and the P proof mark stamped on the left side top of the barrel chamber and nearby on the left top side of the receiver. No sling swivels are noted on US riot guns in the Vietnam era, but there were some Ithaca 37 riot guns made on contract for the South Vietnamese with factory sling swivels. However, the Vietnamese contract guns were not believed to have had US military markings and their serial number range is known to have been from 50,000 to 71,999.
US contract Ithaca riot guns are distinguished by S prefix serial numbers ranging from S1000 to S23000.
I'd say further research is indicated...