SM420 Identification

[akeane01]

I bought a 4 speed SM420 transmission without knowing the year but had some idea. I found out thru other postings that a 7 digit casting number beginning with 37,38 or 39 fall in the 1964-67 year time frame. You could also tell the year by the code above the casting number. For ex: B1060 means it was feb 10, 1960.. I thought I was all set to identify any SM420 but what i have seen in looking at SM420's is that there are some that have the 1st letter followed by only 3 numbers instead of 4. Mine is L184..

So does anyone know how to decode this to figure out what year this is?

here is a picture..

thanks