I think that it is correct. The little bell and the dots represent the ringing of sleigh bells I was told. The front and rear designs varied by whoever was doing the stripes on that particular day.
I think they were 4.75 x 16. The old size is 4.5 and is not available anymore only if you want the Bombardier ones, but they do not have any tread, so I got these from Coker Tire. They have a good website. The 4.75 will fit your B 12 with no problem. I had to space out the bars on the B 7 so they would fit. They are just 1/4" wider but fit the cleats fine.
Your cable it attached to the transmission. You can check with www.redlinegaugeworks.com 661-259-8891. I have never used them, but their site claims they can make any type of speedometer cable. One thing to note is the transmission is Chrysler and the gauge is 1937 or 38 Ford. They may be able to do a gear reduction in the line or a different gear at the transmission. It does read about twice the ground speed. Let me how you made out.
There is really nothing the same about the B-7 to the B-12. The suspension sets on a set of leaf springs and the 3 wheels pivot together like a set of locked tandem wheels. The center wheel is the pivot point so the ride is more of a jerk to the left and to the right. The front suspension is a vertical mounted coil spring that goes directly down to the ski. It is a firm ride.