Better-contrast photos would be needed for a more definite identification, but I'm fairly sure it's a Byzantine follis, 6th to 7th century (probably 6th, due to the weight).
I don't think it's a slick, just unusually low-contrast; the rare kind of coin where a pencil rubbing would probably have helped.
Even as is, I think I can see traces of ANNO on the left of the M and a X and other (unreadable) digits on the right of it. On a photo with better contrast I could probably try to read the year. As it is I can only say that the year is at least 12 (as already expected if this is a Justinian I coin).
EDIT: let's be honest - I see "with an M on one side" and I think "6th-7th century follis". It's an extremely common coin series (perhaps the most common among medieval European coins), and the huge M is indeed rather conspicious.
Aiglet7 thank you for the link the scan doesn't show much detail but it does look like the Byzantine Empire Justinian 1 ( 527 - 565 ) any idea what it may be worth ?
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