I have seen a green mark on the inside of the front of the machine on every 301 I've had the chance to look at (okay, it's only 4). When I open the faceplate and look inside the section where the thread take up lever and other parts like the needlebar originate, there is a green splat of paint. It's in the top right corner. It looks kind of like green nail polish.
When I got my first 301 I thought it was someone's personal mark on their machine, but since I've spied it on every other 301 I've had a chance to see in person, it makes me wonder what it stood for. Does anyone here know?
I'm also curious to hear from other 301 owners about if their machines have this mark.