That trailer is amazing. Sorry for the loss of your grandparents.
etrailer.com has some baby moon hubcaps for about $17 each.
Tough to say exactly when the split rims went away. Light trucks were supposed to have stopped using them as early as '54 but that may have been as per the manufacturer. It's even tougher to say when Airstream quit installing them. I've seen lots of windows that I thought were "out of use in the 50's" but wound up installed as late as '65. That was on a 35 foot special order for the Southern Pacific Railroad. Windows, however irrelevant to this conversation, are a good example of the way Airstream would use things that just hadn't completely disappeared from their "bench stock".
If the rims are the old 14.5 x 7 inch variety I'd get rid of them. Tires are hard to find.
I've got a '59 that had single piece 14.5's on it that may have actually been original but I replaced them with 15's that have as close to a zero offset as possible. I think mine may have had an eighth of an inch offset or something like that.
Thanks for sharing Pictures of your trailer. The first airstream I really looked at was a '61 Sovereign
. I still kick myself for not buying it.