Using the information I found on DIY Ford, I created the following image which represents my attempt to decode the tag I found on the rear axle in my '76 F-150 LWB 4X2.



I think that I have figured out the date of manufacture, ratio and presence of a limited slip differential. Please correct me if I am wrong.
I'd also be interested in learning what those unidentified codes (L2 and 388-D) refer to.
Of course it is entirely possible that this tag doesn't belong with this rear axle so I probably also need to learn how to cross check somehow. The warranty tag isn't correct due to the driver side door being swapped out at some point in the past. If the warranty plate cited an axle code of H4, that would corroborate the 3L25 on the tag. The VIN doesn't help directly but a Marti report based on it very well might. Perhaps Number Dummy can shed some light on these other numbers.

NOTE 1: The letter "M" translates to 12 because Ford did not use the letters "I" and "O" for fear they would be misread as "1" and "0" So M is the 12th letter of the alphabet by Ford's reckoning and, so, indicates December.
NOTE 2: If you type 388-D without the dash in this forum, an emoticon appears in the post instead of the intended text so that's why I added a dash that does not appear on the axle tag.

 

bump for replies

 

The "L2" actually needs to be read as "WFE-L2" this is the axle model number and is needed to get the correct parts, there are a lot of different 9" Ford axles. If you go to the Ford parts catalog and, in section 40, look up WFE-L2 it will tell you that it is a Ford 9" with a 3.25 ratio and (here's the important part) use parts list 184 to look up the correct parts.
The 388-D is the plant code that manufactured the rear. I don't know if there is a list that breaks down the code to ID the particular plant.

 

Thanks Mike for completing the puzzle. So, how do we know the decade of production? Given the long history of the 9" ford rear axle? It could have been 65, 75 or 85, no?

 

Mike didn't notice, but I did...Now it's complete:

5MB = The 2nd week (B) of December (M), 1975 (5).

If B stood for the 2nd day, what letters could be used for the 27th thru 31st days?

 

On the identification tag, Ford didn't identify down to the day of manufacture/assembly of the 3rd member/rear end. It was listed as year, month and week.

The "B" indicates the week within the month that it was assembled.

An "A" would indicate the first week of the month. "B" would equal the 2nd week, "C" the 3rd week, "D" the 4th week and for months that have 5 weeks in them, this would be indicated by an "E".

 

Yup, went right over my head.If that happens much more, I'll end up bald.

 

ND, how do we know it was 75 and not 65 or 85? The 9" was in production from 1957 to 1986, no?

I will update the image once we have gotten a consensus or something close to that.

 

There's no rear axle ID tag code of WFE-L2 listed for 1957/72 or 1980/87.

Pic from 1973/79 Ford 100/500 series truck parts catalog:

 

So here, thanks to Mike and ND, is the corrected graphic on decoding the axle tag on dents with 9" rear axles.



The top line gives us the date of manufacture:
WFE-L2 refers to 1973-79 (from 1973/79 Ford series truck parts catalog - one of many listed)
5MB: The 5 refers to the fifth year, M refers to December and B refers to the second week of that month.
Thus, the date of manufacture is Second Week (day 8-12) of December, 1975
The second line has three data points as follows:
3L25 indicates a 3.24:1 ratio with (L) limited slip or Traction-lock.
9 refers to the diameter of the ring gear which is 9”
388-D refers to the factory code (no known lookup for this)

 

Fixed it for you.

 

Thanks NumberDummy