Thought i would share some research regarding the front CV axles on for this almost identical last generation of the G37/Q40 sedan.
I have a 2015 Q40 and found my front right boot had teared after being on swift lowering springs from 10K to 50K miles.
So i call infiniti for a replacement Cv axle and they tell me the part is discontinued with no suggested alternative. Strange.
Because of my low miles and early catch of the torn boot I decide to order inner and outer replacement boot kits with the intent to simply replace the boots.
$200 for replacement boots vs $200 for an aftermarket axle. Probably stupid but its what i first decided.
After reading about the headache of replacing the boots vs replacing the axle i found the correct part on Ebay new and offered $400 and it was accepted so now i have that incoming as well.

With hours spent trying to figure out the stupid part number thing here is what i found.

INFINITI DIRECT SEARCH
The 2013 G37 Sedan lists front CV axles as
39100-JK500 (right), listed as fits G37 only
39101-JK500 (left), listed as fits G37 & Q40

2015 Q40 Sedan lists front CV axles as
39100-JK50B (right), listed as fits Q40 only (however it is discontinued and not available) with no alternative
39101-JK50B (left), listed as fits Q40 only (however it is discontinued and not available) with no alternative

Infiniti Part Deal Lists different
39100-JK50B (right), listed as fits G37 & Q40 (however it is discontinued and not available) with no alternative
39101-JK50B (left), listed as fits G37 & Q40 (lists as available to buy)

You can find the cheaper Value Alternative Brand part numbers, but good luck with that.
Infiniti site will not offer these typically but you can search directly for them.
C9100-JK50KNW (right), listed as fits G37 only (note infiniti parts deal cannot confirm fitment for G37)
C9101-JK50KNW (left), listed as fits G37 only (note infiniti parts deal cannot confirm fitment for G37)
A word of caution, if you are searching Infiniti site for front axle and it offers you the value alternative, it is the rear axle, not the front, every single time in my experience.

Its is strange that the newer part 39100-JK50B is discontinued but the old part 39100-JK500 is not.
With all this I am left to conclude that all of these front axle part numbers are interchangeable and the part number system is just a mess.
I could be wrong here but I believe that is a safe assumption.

Saved some research here.
CV Boot Repair.docx

Questions:
Anyone rebuilt front axles before? Only installing new boots. Is it worth it?
Anyone want to buy a 50K front right axle with 2 new infiniti boot rebuild kits