I believe the ford/nh pto stub cap is a standard item that hasn't changed much since the late 50's when the 1 3/8 pto became standard. And really.. the only change was length.. the threads are still the same... for instance, the plastic cap on my 1997 NH will screw into the cap holder on my 1952 8n.. though it is big, as the 8n had the shorter 1 1/8 pto stub, etc.
NH should have the plastic pto cap for your tractor.. I'm really surprised they didn't have one on when you bought it.. I would think that was a'safety' item. If the price is over 12 bucks, get a universal ford cap from tractor supply co.. they are aluminum too!.. not plastic.
I like the aluminum ones for durability.. though the plastic one i got 'free' is working.. etc.
Soundguy