I have a wheelchair that was built in California...special order...because I'm special...but when I got out of the hospital I had problems with the brakes not holding...so I tried to tighten the brakes.
At first...due to the drugs I was taking and other factors...lighting et al...I thought the heads were torx...so I went through trying to find my torx..and managed to almost strip the heads....then I recently discovered that they were, in fact, Allen Key heads...
So...I tried to find the right one...looked very much like a 4.5 so I went out to buy one...but nobody actually carried that one as a stand alone...
Finally I took one of the bolts out and brought it to the store...and...it turns out that these bolts are all 3/16" because the chair was made in California...
What a PITA...but, fortunately, most stores still carry the SAE sizes and I was able to buy a cheap set...that I will use maybe once every two years...or so...
I don't mind them usually...my Mountain bike had all Allen Key bolts and it held together pretty good over the years I was riding it...
Easy enough to strip the heads...especially if you try using a torx wrench on them...