Fresh Air Hose Base Seals (or as VW refers to them - Fan Housing to Heat Exchanger Pipe Grommets) are the rubber seals that fit around the connection tubes that stick out through the rear engine tin from the muffler. Their obvious importance is to prevent hot air from the exhaust from getting into the engine compartment, much like the engine seals. VW used them on all fresh-air fan shroud equipped upright engines (1963 - on). During this time these seals changed style twice though, to the best of our knowledge, they are completely interchangeable. From 1963-1973 the 113119585A seal was used. During the early 1973 production (at least on US-export models) - most likely around late 1972 / early 1973 - the new thinner 113119585B style was introduced and used through the end of production. When the 113119585A seal was discontinued, VW superseded it with the 113119585B for all years.

If correctness is what you're looking for, the engine breaks for the seals are listed below.
113119585A
AE code engines - all
B code engines - all
F code engines - all
H code engines - all
AH code engines - through AH0075453
AK code engines - through AK0113790
40hp engines (starting with 3949223 or stale air converted to fresh air) - all

113119585B
AH code engines - from AH0075454
AK code engines - from AK0113791
AJ code engines - all
AM code engines - all*

* - AM engines were only used in the Things. Per VW's Thing parts book only the later style seal was used, however it's plausible that early 1973 production still used the earlier seal and the parts book simply omitted this information.

Interesting footnotes:
1) If you're using an aftermarket exhaust system that forces you to run an aluminum fresh air hose from the shroud to the heat exchanger directly, you usually can't use the seals. The seals are only made for the muffler connection tubes to fit through and not for the hose itself. We would recommend in these instances to tape up any gaps between the hose and the tin with an aluminum heat-resistant tape (normally available at most hardware stores in both black and silver colors near the furnaces and hot water tanks).
2) Many vendors not only have the applications for the seals wrong but also the part numbers on them. For some reason we keep seeing 113119571 listed as a part number but this is actually the earlier base seal for the smaller pre-heat hose.
3) When VW increased the size of the preheat hose in 1968 to 50mm, the fresh air hose base seals were used for this hose also. They changed to the thinner style seal at the same time the fresh air hose did.

Is our info wrong? Please tell us!