When I see Mavic Batteries on sales sites I either see them described as "Part 25" or "Part 26".  

It seems that Part 25 batteries are DJI part number CP.PT.000587

and Part 26 batteries are DJI part number CP.PT.000565

They seem to have an identical spec. (mAH voltage etc.)

Does anyone know what the difference is?

And what does "part" refer too?  It seems to bear no relationship to the MPN.  Is it conformance to part of specification?  
Is the part 26 newer (even though it has a lower MPN)?

I'm intreaged to know.

There have been or still are 2 mavic battery manufacturers - mine were made by Dongguan Amperex Tech - other users have mentioned that their batteries were produced by another company - perhaps this is the reason for differant part numbers for the same item?

We have different battery suppliers, but no difference in function, no worries.

Good to know, thanks for bringing this up.