I just looked
I have some Federal 205's and 205M's, I just took one out of each box,and I can not tell the difference.
I also have a few left when the "M" came in the pretty white box, They also look the same.
For what it is worth, for the past 3 years,I have been shooting the 205's that come in the blue-gray box marked "Champion". These are just regular 205's.
The new Benchrest Primers,(205M's), are in that brownish red container.
The only reason I bought the 15,000 "Match" primers is that is all that was available a while back during the big primer shortage of '07. Forced to pay 30percent more for a different colored box........jackie