Ernie, right after I purchased my 337G in September, the rear probe failed on the way home. My mechanic replaced it with this probe:
Aircraft Spruce ALCOR BY Tempest EGT/TIT Type K Thermocouple & Clamp 3-3/8 Max
My airplane has the optional Cessna Fire Extinguisher under the seat. No age, date, or gauge. So, I replace it with a like-sized bottle. The bracket was different, but my mechanic changed it out in 15 minutes.
I live in a small town, so I took the old bottle into the fire station and discussed it with them. It turns out there was a young Air Force firefighter veteran! He helped me chose a replacement through Aircraft Spruce:
H3R Fire Extinguisher Model A344T.