I am working on a Mercury Model 500, 50hp, SN 3094977 it needs a coil, the original replacement part number was 332-2983A8, I am told this was superseded to 332-4895A6. The question is will a 332-4895A9 be a suitable replacement ( I have found one of these)?

It doesn't show up in the supersession chain,
You are looking for one of these numbers;
332-4895A6
332-2983a2
332-2938a8
332-2938a20
all of which seem to be NLA
You might look into CDI/Rapair
or Sierra