I occasionally spot a guy sportin' one along Whiskey Row, and when I was still in SoCal I'd see a few guys at the Rock Store wearin' 'em, and also of course at some of the vintage motorcycle meets in SoCal which I'd attend. But yeah, you're right. For the most part it's a rare occurrence to run into someone wearin' one.
Saaaaay, maybe it's 'cause the guys who own a Triumph T-Shirt are in a hurry to go about there business out there, and so they don't wear 'em anymore 'cause they know whenever they do they'll always be delayed by some geezer who'll spot it and will wanna talk their ear off about their old Bonneville they had before they:
(a) Went off to Vietnam
(b) Got married and the wife said 'no more motorcycles 'til the kids have moved out'
(c) Lost everything because of a severe drug/alcohol problem back in the '60s/'70s/'80s
(d) All of the above
(...just a theory!)
