It's still not a spacer, it's just two different ways of attaching the drum to the hub, called outboard and inboard drums.
Outboard the drums slip over from the outside same as a wheel fits on the hub.
Inboard drums are on the inside with the studs pressed through drum and into hub.

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-