I found this info on a “thegearpage” thread, answered by Southbay Ampworks:

[quote]what can you tell me about the black back which says marshall rola celestion T2876?
That should be a G12-80 speaker. 50 oz “H” magnet, 444 55hz cone, although I have seen it with a 1777 75 hz cone as well when reconed improperly. I have two original T2876 speakers here, both with 444 cones (same as used in a Vintage 30 and later model G12H30 55hz speakers from the mid to late 70’s). One I’ve owned since day one, as it came new in my Randall RG-80 I bought new in 1982. The other is a BN21 date code, meaning it was made on February 21, 1980.
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I also know that these drivers were fitted in the late ’70s – earl ’80s JMP4140 “Club and Country” 2×12″ combo that is very occasionally seen on eBay for sale. Each driver has an individual impedance of 15ohm, by the way. Your cab is probably standard Marshall practice; 2 wired serial and paired up in parallel to give a cab of 15 or 16 ohm overall impedance.

Arent Rolas 70’s greenbacks? My friend had 2 in a 2×12, not worth bundles of money, but a bit more than the usual £40 greenbacks.
Good speaker.