Busy day today but I managed a few minutes of record digging and found that we started using the GR50 GPS Receivers in the 997 units in August of 2008.

The GPS Receivers are marked as the following on the top:
GR16 states: “GPS ACCESSORY SYSTEM” under the “HUMMINBIRD”
GR50 states: “AS-GR50 GPS MODULE” under the “HUMMINBIRD”
GRHA states: “AS-GRHA HIGH ACCURACY GPS” under the “HUMMINBIRD”
All of these have a dark gray (looks black to most folks) gasket between the upper and lower housings.