Closing the loop on this for any future readers.
Managed to obtain a master parts manual spanning 46-72 medium duty.
The 'felt washer' also known as the Packing - Pinion, Seal that is referenced above and appears in the '63 shop manual and is referenced in the parts manual as:
- The '55-'70 15000 lb & 17000 lb Two speed rear axle group
Group 5.469 PACKING - Seal, Pinion PN:3653617
- The '60-'72 Series 60 15000 lb Two Speed Rear axle group
No felt seal present
The Oil Seal Retainer Gasket
- Group 5.476
'61-'66 Ser. 50-60 w/15,000 lb axle PN: 3758222A.R.
Diff Cover Gasket
- Group 5.3969 GASKET, Axle Housing Cover-Cork
'60-'66 Ser. 50-60 w/15,000 lb axle PN: 3711464A.R.
And in case it is needed
Axle housing gasket
- Group 5.508 GASKET, Axle Housing to Differential Carrier
'64-'66 Ser. 60 w/15000 lb axle PN: 3762399
None of these gaskets are manufactured anymore but apparently occasionally pop up as NOS (though very rare). Your best bet is to just use liquid gasket maker and keep an eye on things, unless someone has a better suggestion.
As to the 'felt spacer' I referenced above, I have two instances of this axle in the parts manual with one calling for its presence, and the other not. Given the lack of one on my particular axle during teardown and the indications that appeared that I was the first one in there, I assume its presence is unnecessary and thus will reassemble as it was originally assembled.