I don't know if by '68 they were still offering the "Lifetime Warranty" or if you still have one attached inside the trailer usually in a closet (we do on our '64), but they will have the VIN # on it if you do.
Our '64 has the VIN stamped on the tongue too, but you're right A8108 doesn't seem to fit. Ours is right up against the body on the curbside part of the "A-frame".
I think I have read somewhere that it might be on the axle somewhere...maybe someone else can confirm this.
I'm surprised your trailer doesn't have the Caravel
plate by the door...that's usually where the VIN # is stamped.