Had a tooth come off on a 300C main transmission ( looks just the same as the pix), only knew as there was a clicking on rundown. AAIB looked at the ring gear and came to the conclusion that it was bad manufacturing. SAC werent interested as they said I had misstreated the ac !! Which led to a very excitable phone call between myself and a certain director of SAC.
Other possiblity is corrosion espically if the machine has stood idle and running on mobile 254. 254 seems to strip off the case hardening letting corrosion in. Make sure when the box is overhauled it goes on to Mobil shc626