As illustrated in the pic the code inlcudes the date (week and year codes) in addition to code identifying the manufacturer. Typically this will be 137 (CTS), 134 (Centralab/CRL), or 140 (Clarostat).
Yes, even if the pot is stamped "Dimarzio", the EIA code is correct as Dimarzio never actually made pots, they outsourced them from manufacturers, typically Centralab and sometimes Clarostat.
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Remember that a potentiometer could have been replaced so it is always best to look at a few of them.
My friends got a mustard coloured Distortion with a block logo and he thinks its late 1970's but he's not sure.
Note that this does not mean a Muff with a 1978 pot was actually made in 1978, it just means it was not made much earlier than 1978.
The Muff could have been made months or even years after the pots were purchased as it is known that Electro-Harmonix bought the pots in bulk, as did Fender, and other companies at the time.