I have a Model 1960B 4x12 cabinet that I purchased used around 1980, and I'm trying to find out what year it was manufactured (see photos).  The serial number is "3479".  The speakers appear to me to be the original Marshall-labelled Celestion speakers, part number "T3120".  Three of the speakers have a stamp near the part number stamp that reads "AM8".  The fourth speaker has a stamp that reads "CM20".  I assume these are likely Celestion manufacture date codes, but not certain.  Only one of the four speakers has a stamp that reads "15 ohms" (but using the omega symbol).  The speakes are wired series-parallel.  According to the nameplate, the cab is rated at 16 ohms (apparently actually 15 ohms), and 100 watts.

Based on the excellent condition of the cab when I bought it, I am assuming it to be mid to late 70s vintage, but would like to know the exact year, if possible.  I saw a post on another forum that claimed the OEM "T3120" speakers are actually Celestion G12-65 speakers, but simply OEM-labelled for Marshall, and that they only came that way preinstalled from the Marshall factory.  Is that true?  From the Celestion website, I see that they still sell "Heritage Series G12-65" speakers, and they are available in 8 ohms or 15 ohms.  The new G12-65 speakers shown on the Celestion website look the same, down to the round mesh vent in the center of the magnet assembly.  The above details would seem to confirm that the T3120 speakers are indeed 15 ohm G12-65s.  If true, that implies that the cab will actually handle up to 260W (not much risk of blown speakers with a 100W head).

I would greatly appreciate any help that anyone can provide.

Cheers,

James