Transmission Output Shaft Question

Why is there both the boot and non boot for the later ones.
It might have been a factory thing. When I owned the '74 Challenger, the transmission had never been out(I was the 2nd owner) and from sitting for over 20 years it leaked from every orifice, so a reseal was necessary. The extension seal was the booted type. We carried both in stock because, some rebuilders wanted the non boot style, others would only use the booted style.
Picture of the Challenger. Bought in 2002 off the original owner. 32000 or so miles at time of purchase. Rallye package, 360/727/3:55 sure grip. Other than doing a major tune up, new tires, new battery and the transmission reseal and of course washing and waxing it, it didn't need much. Never driven (according to the seller) but was run on a regular basis in the driveway. It was parked in his garage in 1979.

My Challenger.png