Darrel,

I found this regarding the 4th pair.

THIRD PAIR (aa-xx)
Each field can be further divided into (24) 5 minutes (0.083333 degrees) longitudinal
by (24) 2.5 minute (0.0416665 degrees) latitudinal zones.

The first character encodes the longitude and the second encodes the latitude with
letters “a” thru “x” (refer to example 3 below).

Example 3:
Longitude = 0.338038 {Refer to example 2}
= 0.338038 / 0.083333
= 4 remainder 0.004706
= e
(location in 5th zone)
Latitude = 0.664048
= 0.664048 / 0.0416665
= 15 remainder 0.0390505
= p
(location in 16th zone)

FOURTH PAIR (00-99)
Each field can be further subdivided into (10) 30 seconds (0.008333 degrees) longitudinal
by (10) 15 seconds (0.004166) latitudinal zones. The first character encodes the longitude
and the second encodes the latitude with numbers “0” thru “9” (refer to example 4 below).

Example 4:
Longitude = 0.004706 {Refer to example 3}
= 0.004706 / 0.008333
= 0 remainder 0.004706
= 0
(location in 1st zone)
Latitude = 0.0390505 {Refer to example 3}
= 0.0390505 / 0.004166
= 9 remainder 0.001556
= 9
(location in 10th zone)