The second photo is of the underside of each forearm. Note that the later vintage forearm on the right has an extractor protruding above the base of the iron. This device raises the shell extractor when the gun is opened so shells can be removed. The forearm on the left does not have this feature because one of the two functions of the joint roll check is to raise the extractor for shell removal.
The last photo is a comparison of the inside knuckle area of the two fore irons. The later fore iron shows the hollowed out area Jent referred to as a "figure eight"; this milled area allows for rotational movement of the cocking rods. You will also see a pin just below dead center of the iron. This pin is depressed by the knuckle of the frame upon opening and activates the previously noted shell extractor. The fore iron on the left is void of this machining as it is not necessary given that the joint roll check raises the shell extractor.
I'm not sure of any reasons why the roll check joint was an advantage over the later cocking design; but if the joint roll gets out of position, barrels can be very aggravating to reattach. Also, should someone slam the barrels open and break the "ears" off the roll, the shooter is presented with a major problem. I suspect the roll joint check feature was eliminated 1) as a cost saving feature, and 2) it didn't allow for the installation of auto ejectors. Col Brophy had this to say about the joint check "Although a patented feature of the LC Smith shotgun, it is my personal observation that the rotational movement of the barrels on early Smith guns is first stopped by the cocking rods and forend iron"; and further "If the patent claim was valid, the feature would surely have been retained". Hope this helps some.