My Simson boxlock 16 ga sxs proofed 2/62 has very similar dimensions to those measured by Nitro. My Simson boxlock 12 ga sxs proofed 8/50 (that is styled exactly the same as the 16) has the following dimensions:
1.12" firing pin center
2.29" across the side clips
1.61" across the action at the bolster
.95" depth of the action

I believe the 16 is built on a true 16 ga frame.

Forester