C9837 is reported in the listing as B4 and C4 of 8 Sqn, wrecked 18 Jul 19. Photos said to be of this incident on page 109 show it coded B4, so it was either C4 and then B4, or the reference to C4 is incorrect.
I agree the code C4 seems a bit dodgy Can one assume that the letters in front of the digits indicated their flight status? B being B-flight and thus C being C-flight? In such a case it is not impossible the aircraft may have been coded C4 before being wrecked as B4, but that is of course pure guesswork
On Page 144, there is a photo of F4621 coded J, captioned as being with 28 Sqn at Ambala in 1928. In the individual aircraft listings, there is no mention of this aircraft serving with 28 Sqn.
No idea how strict the markings in each Squadron was made/followed by those in charge, but looking at F4621 'J' (top photo on page 145) and the one coded D (just below on the same page) their unit markings differ quite a bit. Aircraft coded D has the unit (28 Sq) painted on the fin (digits 28) and the code J painted in front of the cockade, while F4621 lacks '28' on its fin and also has its code painted behind the roundel.
This is of course way out of my comfort zone so no idea if that actually means anything. Perhaps if Eric Harlin is still with us, he can make some comment about it via Phil Jarrett?
The entry for E2458 refers to it being marked as "8" with 88 Sqn in 1918. A photo on page 246 shows it to be coded "9" at the time of its loss on 3 Mar 19. So it was either recoded, or the reference to "8" is a typo.