Electric speedo question

[C15_Sierra]

Hey guys - I snagged a Burb/Blazer electric speedo for my 5.3 swap a while ago and I'm getting around to installing it in my cluster - I noticed in other pics that when installed there is a box behind the cluster - is that the calibration box? I bought the speedo off eBay, and it did not come with the box.

Questions are:
1) Am I right in assuming that's the calibration box? Or is it for the cruise control like the ones on mechanical speedos?
2) Where can I get a calibration box?

Speedo in question:


Circuit board for the speedo with 4 pins, presumably to go to the cal box:


Thanks for the help

[C15_Sierra]

OK in doing a little bit more research, apparently the PCM will be a "calibration box" so to speak, now the question becomes:

How can I wire up the electric speedo to work with the PCM?

Thanks!