Okay - let's see if we can logically get the number...
This is the list presented in the books...
This is the list in alphabetical order
This is the list in Numerical Order...
As you can see there is no rhyme or reason to their naming convention. It looks like there were some, and then lost on another.
I have some assumptions based on the listing. I would probably rename nearly all of them to the following:
Here are the three basic Power Armor based on Mode
NG-X (Ground Based Power Armor)
NG-NX (Naval Power Armor)
NG-FX (Flying Power Armor)
NG-EX (Exoskeleton Power Armor)
Here are the Power Armor that has a specific function or Department
NG-XG (Gladiatorial Power Armor)
NG-XP (Police Power Armor)
NG-XF (Firefighter Power Armor)
There are two others that need to be represented with are the UELX and the MRU, so I left them alone. Because the numbers aren't sequential or were duplicated I made some additional changes other than the NG-X44 repeating.
NG-UX (Ultralight)
NG-MSUX (Mobile Support Unit)
I'm not really keen on these. Nor am I really keen on the idea that the Gladius is now a PA. But I am not really trying to reinvent the wheel, just modify it into something rational.
I did find it funny that the XP54 Enforcer was a Police Unit Armor (Car '54 Where are you) jab. I would have loved to see the Firefighter with the number 451.