Richard : I agree, that in "today's world" with our cars, chances of this happening are slim. However, "back in the day:, when our cars were sold as "Daily Drivers", Chances of this happening were much more prevalent. In the mid 60's and early 70's, I can't tell you how many times I got "caught" in my 57 Retractable with the top down, and one of those Midwest showers popped up.o frequently, that I kept a towel under the seat just to dry things off after I put the top up. However, once I Restored it in 1976, only once or twice do I remember getting "caught", and one of those was at a National Meet. So, Bob's first sentence in his Reply above, makes good sense.
There is also the case of being out the day after a rain, and stepping in a puddle just before entering your car. But this case seems to reinforce Bob's second sentence, which makes the case for, "why not every Model, not just convertibles" ?
In a quick look at my 66 and 67 Electrical Assembly Manuals, I did not see the rubber cover noted, It does, however, appear in the 1965-67 Parts and Accessories Catalogs (single year MPC's) and the 1960-68 and 1965-72 MPC's ; all list it for only Model 76 Mustang - Convertibles. The 1965-72 MPC shows it for all 1965-72 Mustang Convertibles.
Bob