https://www.dropbox.com/s/4z281z45sc..._6505.MOV?dl=0
Give this link a try to see the video of the gear teeth damage, to my eye the mating gears appear fine.
"I have never seen an M40 case with the higher part for the filler plug."
Exactly, but for some reason my gearbox is tagged M40! I've sent this question to Volvo archives to see if they can explain as there records indicate the 18345 Type designation as an 1800(18) with overdrive(3) for the N American market(4) having a M41 gearbox(5).
I haven't torn the thing apart yet, just looking down with the shifter cover removed as when I drained it of oil one tooth dropped out and then after opening the top cover and laying it on its side a second tooth dropped out. The mating gear is easy to look at and doesn't appear damaged.
What I do know is the second gear with the teeth missing had 20 teeth.