21 Feb 2019 ::

Although untested [Now tested. See below.] at the moment, everything EXCEPT magazines looks to be interchangeable between the GLOCK 43, 48, and 43X.   The slide from my GLOCK 48 fits perfectly on to my 43.   Barrels; same situation.   Internally, they use the same trigger, locking Block, trigger mechanism housing with ejector, connector, yadda, yadda, yadda.   Naturally, you can’t put the short barrel of either 43 into the 48 slide.   But, a 48 barrel in your 43 ::  that works.   A 43 lower for your 43X slide ::  technically it works, but why limit yourself to 6+1 ???  (Unless - - you are stuck in some Anti-Freedom state.)
It is the magazines that are the conundrum.   The 43X and 48 have their own (10 round!) magazine design.   The original 43 magazine (6 round) is narrower by 0.040 of an inch…   Thus, the magazines of the two new models (43X and 48) do NOT work with the older 43 frame.   And, vice versa…

The 48 even fits in my 43 holster (CrossBreed AIWB).   Yes, an inch of slide/barrel sticks out the bottom, unprotected.   I may super-glue a leather tab in that location.

The following may be for a different article, but my bottom line advice is this :: If you do NOT already have a 43, then go with the 43X as a new gun.   (Better concealability and 10+1 capacity.)   If you DO already have a 43, then augment it with a 48.  (Your 43 upper on a 48 lower IS a 43X; and you have the flexibility to swap to the longer 48 upper at any time.)

I will confirm that mixing components works just like 17-34-17L, only in miniature, as soon as the weather becomes tolerable !!!

ADDED 16 JUNE 2019 :: All of the above has now been tested, and confirmed to function perfectly (with about 50 to 60 rounds in each configuration). While I had the 43 upper on the 48 lower (43X configuration), I also put the 48 upper on the 43 lower. The latter is definitely an odd duck :: nearly a 19 profile with 6+1 capacity. Massachusetts special? 43L?

Thx for reading.  This is a work-in-progress.   Check back.