No. 405 A.R.F. (…) Aircraft Repair Flight

Are you sure this is correct? I have 405 ARF as Aircraft Reception Flight. I think repairs were handled by RSUs (Repair and Salvage Units).

what could an Aircraft Repair Flight do which an M.U. couldn’t

What an MU could do depended very much on the actual MU in question. Some MUs handled repairs, some did overseas crating/uncrating, and some were just storage units without staff or equipment for any kind of repair/assembly/disassembly operations. “RAF Flying Training and Support Units” by the late Ray Sturtivant and John Hamlin is a recommended source here.

The impression, maybe, but it isn’t right. What it means is that the Air Min had already decided that, since there was no real difference in performance, it was an LF.IX whether it had a Merlin 66 or a 266- much as an LF.V could have a 45M or a 50M or a 55M. But there were some installational differences, and possibly (I’m still a little vague on this!) some differences in tool-kit required, so those uppity folks that actually had to deal with such issues pressed their point.

I don’t really think it was for technical reasons. Differences in engine installations of the various Mk Vs were no less than those between Mk IX and XVI. But, Packard Merlins were delivered under Lend-Lease Act which put important limitations on what the Crown was able to do with them if not used by the RAF. Note that after the war Mk IXs (all-British made) were sold worldwide, while Mk XVIs (with the Lend-Lease engines) were retained in RAF use. The only country to actually buy ex-RAF Mk XVIs was Greece which used a lot of the US military funding anyway.

Isn’t the ‘b’ cannon port part external to the wing skin? Looking at some photos recently I thought I could see it as the outer layer. The ‘c’ piece is internal.

Exactly! The “Mk IX wings built with Mk Vb front castings” theory seems to have been born from the “Mk IXB” designation in some documents. However, “Mk IXB” had nothing to do with the armament installation. It was an ad hoc informal name given at unit level to the “new” Mk IX as opposed to the “old” one before the official designation of “LF.IX” was introduced. I can’t see a reasonable way of using the Mk VB-style cannon chimney pot with the Mk VC-style cannon installation.

We’ll see in a couple of weeks, I guess.

I think it’s obvious that the terms ‘Germans’, ‘Americans’, ‘Russians’, ‘British’, ‘French’, ‘Japanese’ etc. cover all of the people of those nations and, naturally, today mostly include people who were not yet born in WW2, and therefore blaiming them for any attrocity, or praising them for any heroism of their compatriots from those times is not justified. However, it is an interesting thing that when we discuss WW2 we seem to have no problem calling the Japanese, Russians, Yanks, Brits, Italians, Frenchmen or Poles by their nationality (whether it’s their brave deeds or nasty things they did), but when it comes to the Third Reich it’s usually ‘Nazis’ rather than any particular nation.

Having been to Zagan for the anniversary celebrations last Sunday and Monday I also noticed that the nationality of the ‘Nazis’ is virtually never mentioned these days.

no serial numbers are recorded in the log book, just individual airplane codes. (…)
I guess we will never know the serial of the Spit he flew with those markings.

Quite the opposite!
1st, by checking pre-17 September entries for ‘S’ in his log book and cross-checking with the Squadron ORB you might be able to ascertain the serial of the Spitfire he flew on that day (I guess it’s safe to assume it has not been recoded DV-S on 17 September morning, but had flown with these codes for a while before that).
2nd, by checking post-17 September entries for ‘S’ in his log book and cross-checking with the Squadron ORB you might be able to establish the serial of the Spitfire in the photo. W3893 is a candidate, being a W-serialled Spitfire delivered to 129 Sqn in the second half of September 1941, possibly as a replacement for the DV-S damaged on 17 September.

Any chance you might post the picture here?
Also, did the Spitfire suffer any damage during the operation on 17 September 1941? P8707 was, apparently, damaged cat. B in combat on that date.