An AX-15 would be a 23 Spline which IIRC is the same for the NV3500
you're right they are all 23 splines but the tailhousing is different on AX15 compared to NV3550 or NSG370, i remember this when i did my D300 swap i went with the NV3550 as it is deeper at the output shaft, i think with the AX15 you might need a spacer or to remove the seal on the AX15 and use the D300 mating to seal the transmission (so that the D300 input shaft has more room and not use a spacer instead), not sure if there are any issues with the 241OR or not, i have no idea.
EDIT: just did some reading and apparently there are 3 options when going with a D300 and an AX15
1. spacer
2. remove the output seal and use a gasket and rtv to seal between the D300 and AX15
3. cut the D300 input retainer and use a sealed bearing (NSK 6209 DDU for example)
for option 2 and 3 you might need to zip-cut the output shaft on the AX15 a little (or grind) so it won't bottom into the D300 input shaft
none of these need to be done with the NV3550 or the NSG370
the 241OR has a shorter input than the D300 but i don't know if it's short enough to fit the AX15 (they were never coupled together from factory), so you might need to do some measuring to confirm - just from memory i think it might fit