The patina of the OPT end bell covers appears to match the patina on the power transformer end bell covers with a little less rust on the lamination plates so it does look original to my eyes and Obeid Khan knows his Marshall's well so chances are it is original or was the Marshall replacement in the early to mid 70's if it did blow up at one time judging by the patina. Marshall supposedly used the Dagnall C1998 Output transformers from 1968 until 1973 but some amps could have come with Drake models 1209-3 and 1202-99 probably due to if and when Marshall had the C1998 in stock.
Since yours has the Drake 1209-3 PT so more than likely it is the Drake 1202-99 Output transformer.........
Either way it does look vintage correct to the amp, I would just enjoy it unless you really want to identify whether or not it's a C1998 or a Drake model I'm sure someone can identify it correctly.
I found this information for you on the net:
Around 1968 Marshall switched to Dagnall power (T2562) and output (C1998) transformers (four EL34 tubes). From 1969 - 1973 most 100 watt Marshall amplifiers have the Dagnall transformers, although a few have the Drake 1202-99 output transformer and the Drake 1209-3 power transformer.
https://reverb.com/item/32357526-marshall-dagnall-100w-output-transformer-1971
I found this pic of a 1971 Dagnall C1998 Output Transformer there are no ID numbers on the end bells.
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