I've gone back to the minutes of the 13th Meeting of DME W/P Committee for 5 May 1944 which I found in WO229/76/0/1:
3. Austin 3 ton 4 x 4 and 3 ton 6 x 4
DME made a statement with reference to the re-working of the engines of these vehicles which had failed owing to engine trouble due to inadequate piston clearance, on large-scale trials at No.1 M.T.C. WEYMOUTH.
Austin Motor Company is re-working all engines required for vehicles for Assault Force, and the vehicles should be completed by 17 May 1944...
Reserve vehicles of these types are not having their engines re-worked as the defect is only experienced when wading.
Tom, great find!
So:
1. Note that the "wading/piston clearance" problem also affected 3ton 6 x 4 vehicles!
The LATER problem also did

At least, you found that set of minutes from 8/11 that referred
in its title to the K6s as well...
...but the brief 3/12 comment didn't
So we have TWO references to both K5s and K6s....but
plenty in lots of places for K5s being worn, going U/S, being tested, worked on, withdrawnlate August -> September and reissued other B vehicles instead of.etc., etc....
2. If the problem experienced during wading was "due to inadequate piston clearance", then the references to making piston clearance greater that we have found makes a lot of sense.
Yes - inadequate piston clearance
because the cylinder suddenly shrank onto them!

Given that doing something about the pistons would be FAR easier and quicker than doing something about
the engine block!!!
3. Details of "wading failure" must have been produced by No.1 M.T.C. WEYMOUTH - anyone have any idea which branch this would have been; REME, RASC, RAOC?
Tom, I'll see what I can find...there's some articles in CMV that refer in passing to tests on invasion bound kit forthe previous 2-3 years, I'll see if it says
who ran them.
4. So we can probably assume that the 1400 Austins we are talking about are those assigned to the assault force and that were re-worked to give them greater piston clearance - this later lead to further problems with excessive oil use, exhaust valve burning, etc.
Yes indeed. The numbers were already FAR too coincidental
5. Now if only we could find out when the "freezing" order was given to the VRDs.
...and if AUSTINS were given any instructions...or
asked for leeway!...to stop production
while the problems were sorted
Reserve vehicles of these types are not having their engines re-worked as the defect is only experienced when wading
Possibly why you were seeing them in film clips in late September!
Be interesting to know how many
NON-wading, NON-waterproofed K5s were sent to France in the same period...!
Also...scope out the timescale...
...the 13th Meeting of DME W/P Committee for
5 May 1944 which I found in WO229/76/0/1:
3. Austin 3 ton 4 x 4 and 3 ton 6 x 4
DME made a statement with reference to the re-working of the engines of these vehicles which had failed owing to engine trouble due to inadequate piston clearance, on large-scale trials at No.1 M.T.C. WEYMOUTH.
Austin Motor Company is re-working all engines required for vehicles for Assault Force, and the vehicles should be completed by
17 May 1944...
...I wonder on what date(s) the said "
large-scale trials at No.1 M.T.C. WEYMOUTH" took place and the problem appeared???
I'd have loved to be a fly on the harbour wall THAT day, with K5s dropping like...well, flies!

Twenty years ago we had Johnny Cash, Bob Hope and Steve Jobs. Now we have no Cash, no Hope and no Jobs....
Lord, please keep Kevin Bacon alive...