My 56 has Stamped on differential the numbers #3725899 which leads me to believe that it's a early 57-61 that was installed late in 1956. the only other numbers i could find are J222. If I'm correct that cast number means the production date would be. October 22 1962.

Any thoughts on this, anyone ?

The casting number for a 1955 and 1956 differential carrier was 3725899. Improvements made to the carrier in 1957 and number changed to 3743833 and if you purchased a new carrier for a 1955 or 1956 after 1957 you got the 1957 version. Further "improvements" in 1958 and number changed to3743833 and that unit carried back to a 1955 and up replacement. These are raised casting numbers and not stamped.
The date and application code for 1956 would begin with two letters indicating the ratio and four numbers indacating the day and the month. Year is NOT included. These numbers are stamped....
Your 1222 would be Dec. 22 and it would be safe to assume it was made on Dec. 22. 1955.

In 1955 and 1956 we replaced a lot of ring gear and pinions due to howling noise. The noise would peak at 55 MPH on Power Glide cars. To eleminate this problem in 1957 they cast in a litle baffle to get more gear oil between the gears at speed. Also the cars came with a special gear oil (with more sulfur) FOR BREAK-IN AND WAS TO BE DRAINED AT 1000 MILES.THAT IS WHY THE 1957 HAS A DRAIN PLUG. tHE CARRIER WAS ALSO MADE STRONGER IN 1957 as were SOME OF THE INTERNAL PARTS DUE TO h.p. INCREASES.

Gene,
Can't speak for the original poster, but I found this info very helpful.

Yes Bob, you can speak for me. The information was a lot more than I expected. Your knowledge is irreplaceable and has helped so many people. I've printed your response for my personal information and maybe pass it on to someone else.
I thank you once more Gene for the detailed information ! P.K.

I am always happy to share my experiance and knowledge with others as it is useless just laying in my brain.
If it wouldn't be for this site I may have forgotten many things by now.

Gene,
I FIRMLY believe participating in these forums is a positive exercise, to assist in keeping our minds HEALTHY.

My thoughts also. Between the chat site and selling tools to mechanics two mornings a week give my mind a good work-out and would't do either if I didn't enjoy it.

Understood ..... a daily mental workout, with a subject we enjoy.

Now ..... what the heck were we talking about ? ?

One more question ? Went out into the garage yesterday
( snowing )and decided to check the differential to see
what the gear ratio ( 3:55 ) was.
Jacked up driver side rear tire (205x70x15 ) marked the drive shaft then rotated the tire two full turns. The drive shaft
rotated three full turns plus maybe 1 1/2" or 2" at most.
That sounds more like a 3:08 and not a 3:55. I think I might
be just confused, but not sure ?
Just trying to increase my knowledge and understand this.....
P.K.

To check the ratio you need to have both wheels raised or the car on the pavement and one turn of both wheels turning together, then count the turns of the drive shaft.
There was no 3.08 ratio until 1959 348 engine cars.
The factory stamping for ratio I.D. is the two LETTERS stamped in front of the date numbers.
In 1956 the 3.55 used either AB or BB.
Actually on the road your speedometer and odomoter will run about 7% fast due to the smaller diameter of the size radial tires you have.
To install 3.08 gears in the 1956 differential may not be even possible due to the large diameter of the 3.08 pinion gear among other things.

Gene, you are correct about my speedometer being about 7% off that is why I was trying to determine the gear ratio. I have the TH-350 in my car and now I need to pull the driven gear to see what number of teeth are on the gear and go from there.
This is the answer I received on the tri 5 site about checking gear ratio. If it's an open (non-posi) rear, jack up ONLY one wheel and turn it 2 revolutions and count the drive shaft turns. If you can't turn the one wheel with it in neutral and parking brake off, it may be a posi. Then jack both wheels up, turn one wheel one revolutions and count the drive shaft turns.
Thanks once again Gene, We depend on Ya!

Due to the "slop" in the side gears, etc. I would find the one wheel version very inaccurate.

Gene, as soon as the snow is gone and I find someone to con I'll
do it the way you explained. I'm sure it's a 3:55 but I need to do it, Just to do it ......... P.K.

Drive it up here to Wisconsin. We don't have snow (at this time)

If you get a speedo. drive gear with one or two more teeth it should correct the error.

Gene have you considered formally documenting your knowledge? Seems like you contribute to just about all threads here and save the day! Write a technical book?