I have attached four photos of three different trigger groups; top one is from a mixed serial number Model 17 Remington, middle one is from an Ithaca Model 37 w/ serial number 806XXX, and the bottom one from the Magnum Model 37 shown in a photo on the Forum earlier with a serial number in the late 1970s/early 1980s. Also checked my one and only Ohio built gun and it is identical to this Magnum gun.
The Model 17 is/was equipped with a disconnecter as designed by JMB. It is the hooked shaped vertical piece in the photos. On this gun anyway, if you hold the trigger down while cycling the slide, it will not fire and will only reset if you cycle the slide again or simply press the slide release and repress the trigger. All Ithaca 37's are missing this part. My guess is that in redesigning and modifying the gun for ease of manufacturing and to reduce costs this feature (and some other minor deletions & changes) were deemed unnecessary and eliminated. It has to be remembered that at that time (late 1920s to early 1930s) the slam-fire was not considered an issue and was included in a number of gun designs that were being built at the time.
If you look carefully at the hammer of the Model 17 (top) and the earlier Model 37 (middle) you will see a little protrusion out the right side of the hammer which interacts with a notch on the slide release. On the early Model 37s this is the second "sear" that allows the hammer to remain cocked while the slide is cycled and will then drop the hammer when the bolt is in battery (and the slide release is reset by the bolt) without resetting the trigger. In the mid 1970s or so, Ithaca eliminated the slam-fire feature simply by removing the little protrusion from the hammer. If you hold the trigger down on a newly manufactured gun and cycle the slide the hammer simply follows the bolt as it goes into battery and the spring loaded firing pin prevents the gun from firing. The only way to reset the hammer is to cycle the slide again, ejecting the live round in the chamber. It is interesting that the slide release on the new gun still retains the little hook notch even though it is totally unnecessary.
While I have not done it, I think that if you want to disable the slam-fire feature all you have to do is file the little protrusion off the hammer and the gun will act like a new one. Again, it will not reset the trigger and fire without cycling the slide. Based on the price that I understand was quoted by the Factory in Ohio I believe that this all that they do; roughly 0.5 to 1.0 hours work. And, since the 3" Magnum guns use a different hammer, you can not simply replace a new style hammer with an old style hammer to get the slam-fire feature without additional work. As far as I know all the 3" guns did not have the slam-fire feature.
Just some added info for discussion.
All my Mossbergs and Browning's have disconnectors in them. At Front Sight we do what are known as "Trigger Reset Drills." We do them with all types of guns, Rifles Pistols and Shotguns. I am pretty much engrained with trapping the trigger and then resetting it after the gun cycles or is cycled.
When trying to shoot Skeet with my Ithaca, I ran into the problem with the gun not having a disconnector and since I have been trained to trap the trigger I was discharging the gun when I cycled it. I obviously missed the doubles targets.
I don't want to break this habit of trapping the trigger because every other gun I shoot works this way. That is Glocks AR's, Mini 14, Kel-Tec, Browning shotguns, Mossberg shotguns, Even my two Over/Unders(mostly using those in my
survival gear kit) work like this. This took me a long time to get in place and I don't want to fool with it for one gun. I want the Ithaca to have a disconnector like supposedly happened in 1975 !