Some interesting information. I think Lionel downplayed the tendency of auto couplers to open, especially on 072 switches. I handle a lot of prewar trains, selling on the internet, and have seen many modifications to correct the problem. It looks to me as if operators favored running long trains without separation problems over the ability to uncouple between every car. I have seen pick-up shoes, removed, taped with electrical or medical tape, coupler hoods wired or glued shut. I try to put things back to factory, but for my own operating use solve the problem by alternating auto with non-auto coupler cars. For example a boxcar consist would be 814, 2814, 814, 2814. This way only one coupler at each union had the tendency to open on a switch.
The plastic nib was probably Lionel's "fix" to slightly elevate the shoe as it passed over switches and hopefully not make electrical contact.......until the nib wore-down. The raised edges on the 1940-42 coupler shoes (also 1939 2600 series) provided the same fix, until wear brought back the problem.