...VERY interesting reading.

I don't know about the X15 dash timers, but I'm currently making some research and i found some info that might be of interest to your research:

First, a Mercury2 main instrument panel diagram and a Mercury 7 picture of the interior There are significant diferences between the two, the M7 not beeing so complete as the one on the diagram (M2) and appearing to have only one clock on the "timer panel", maybe that is why Glenn had to carry the chronograph on his wrist (?) - Both pictures can be found here

Second, to respond to your question about the scans of the X15 dash...oops!...cockpit :-), there's an interesting picture that has enough resolution to show us that the chronograph used is a likely a Minerva, from the (blurred) logo on the dial...This picture, taken on April 1963 can be found here

Best Regards

Francisco