What is the obvious difference in the 15 and later model T rear ends?
One is that the 1915 carrier was held together with studs & nuts, and the later one with bolts.
The rear axle housings were back to 1906-08 NRS style after a few years with pressed steel and riveted experiments that weren't as sturdy as the earlier style.
The 1915 rear end has smooth backing plates like the older 1909-14 rear end.
The 15 plug is higher on the housing, at 9:00.