I have two that will not fire with the trigger held back --- 37147xxxx and 37153xxxxx both guns have a "disconnector".
The way Ithaca stopped the firing when the bolt was closed and trigger held back was to remove the second sear on the hammer. If the trigger is held back the hammer will follow the bolt forward as it closes.
The hammer was held back by the action release lever and released the hammer as the bolt came into battery. The release lever popped up preventing the bolt from moving back, at the sametime the hammer was released.
My serial number range indicates the guns were made in the first or second quarter of 1975.
Maybe someone has more information to add, I would also like to have more info.