I checked my photo archive and found a pic of the gearbox casting number, it doesn't match:
In regards to how to shift gears, if it's a variation of the NG3 (NG0,1,3 etc) the shift pattern should most likely look like this at the gearbox end:
And, this is how I had originally wanted to set up my shift assembly, but finding suitable u-joints for the linkage was going to cost more that I had hoped:
The shift lever off the gearbox had a cup welded to it to accept a 5/8" spherical bearing, the triangulated piece had a slip fit into the spherical bearing to allow for a change in vertical distance (if that makes sense), and the rod the triangular piece was welded to was free to slide in and out along with rotate in the outer sleeve/bracket. It would have required snaking a shift linkage along side the motor with u-joints. That idea was abandoned and a cable shift set up used instead:
The lever on the side of the 'box was cut and reversed to get the shift cables to the side of the casing rather than right under it. The lever on the side to side splined shaft was salvaged from a Coventry Climax firepump mainly because it was in my box of odds and ends.