decoding the 64-66 1/2 ton Rear end gear ratio

[GMCNUT]

Hey guys - I have a "known to be good" spare 1/2 ton rear end I saved for a rainy day and I am building a 66 short fleet, so I decided to pull the inspection plate and see if I could count the teeth or something to determine the ratio....I could not tell if I was counting the teeth right, so I snapped a picture of the stampings on the ring gear and decided to sling this info at you guys and see what you can tell me? not even sure these markings tell us anything, but willing to try! See pictures below.

Hopefully you can see these markings clearly, but in case you cannot, starting from left to right, is the GM pt # 3817778 followed by an 11 then a 41GM then a 3 then what looks like a date of 64 1 27. on the opposite side of the ring gear it says 2-18.

If this info does not tell us the gear ratio, please feel free to suggest something. My next idea was to wipe a little white paint on one of the teeth and turn the ring gear and try to get an accurate count that way. As for counting the pinion gear teeth, I have no idea how to do that since it sits deep behind the carrier and everything - may have to remove it to tell. Thoughts? Thanks, Steve