Recently I found a service notice for a Bell Sound 6060 amplifier which has been uploaded.  The notice said the circuit change would be effective beginning with serial numbers starting with 29.  Given that bit of information I created a spreadsheet to examine Bell serial numbers and discovered a pattern.  This pattern of rules appears accurate at least during TRW's ownership beginning in 1953¹.  (edit) I have now found two Bell products made pre-TRW and serial numbers appear to follow the same patterns.  The model/serial number combinations were found by examining photos from RM.org, ebay, google images, and one or two came from soliciting users at audiokarma.org.  If anyone has additional combinations to share please let me know and I will add them to the spreadsheet linked below.

Bell Sound Serial No rules:

From LEFT to Right:

1st digit = production run

2nd digit = year

3rd -  6th digits = sequential unit number

Sequential unit numbers appear to be within the context of the production run and year.

Example:

Model 3030 serial number 190220 = 1st production run, 1959, 220th unit made.

Model 3030 serial number 580423 = 5th production run, 1958, 423rd unit made.

One might assume serial number 190220 to have been made earlier than 580423, but now we see that assumption would be incorrect.

Here's a link to the spreadsheet I created:

Google Spreadsheet

1. TRW: Pioneering Technology and Innovation Since 1900, Davis Dyer, 1998

---Matt