Tom, you might be right! I pulled off the old breather cap, which doesn't fit very well, and noticed around the lip of the fill pipe two extensions that go about 1/2 way around. I guess those are for a normal cap's lower edges to hook under when it's twisted on. Your idea about it being a '49 would jibe with the PO's story, except that the 'LBA' stamping indicates 1953, as far as I can tell.
It would also seem redundant to have a breather cap on a filler that already has a breather tube, too, come to think of it.
Thanks.
