You'll be fine Steve. IIRC, the same firmware is used for 561 and 561M anyway. I used the 561_177.mfu when I was playing around with a 561 a couple of years ago.

Lots of differences between 1.3 and 1.77 I think, probably including addition of PLII (though that might have been pre-1.3).

PLIIupdate.pdf

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