The end of my passenger side heat exchanger is missing what looks like a cap.

I'm not sure though and searching for "heat exchanger cap" yields no results so I can't find it at the usual suppliers.

Any help identifying it?

This is the drivers side and it is present (circled in red):

Click to view attachment

On the passenger side it is missing (circled in red):

Click to view attachment

its a dust cap.

allows the heat exchanger to vent out the hot air if not being sent forward.
but keeps muck and dust out of the control box and heat exchanger tubes.

EDIT

is the rhs one missing. passenger side.
could be because of a dealer mod that was done in the 70s for vapor lock.
@JeffBowlsby has an interesting tech note in his library of technical material on his website. porsche suggested getting rid of the dust cap cover and fitting a section of heater hose (Flexi duct) to that outlet and directing the short length of hose rearward.
to direct the heated air away from the fuel pump which was in the vicinity up to 74 models.

its on his website. i have had a look at it and contemplated doing the mod.

if it is that one maybe the dealer mod of flexi hose fell off years ago?

Had the vent cap for vapor lock mod (open-ended hose) but kept it. Replaced it when moved the field pump to the front.

when you look in the parts catalogue its shown as being part of the control box.
would appear you have to buy a whole control box.

but maybe a member here could sell you one separately.

the other possibility is that it is the same as ones that are on many other VWs.
and you could search around on VW parts websites and see if they sell it separately.

Thank you all!

My mechanic did try to reroute with a heater hose but it didn't work. So I moved the fuel pump to the front.

I guess I need to ask him if he still has it.

If not I will followup with your suggestions.

Great community here!

Not supposed to be separated from the main part (see below).
Check with Bruce or the classifieds I'm sure you can find one.