A true spacer that is removable is very uncommon, and non existant on the later model P series you're asking about.
Talking about front axles.
The 8 lug P30 hub is the rotor as well, all one piece.
The 10 lug (5 front) P30 has a hub with the 5/10 lug bolt pattern and a removable rotor, two pieces.
You can interchange between the 8 and 10 lug spindles/axles the complete hub and rotor along with respective caliper brackets (the 10 lug rotors are larger).

What are you trying to do?

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-