Portland & Worcester Rpo, Scott #448 Or 486

I picked up a few postcards, including this RPO. Not a great postmark, but it looks like it's Portland & Worcester from 1921.




The stamp is a rotary perf 10 coil, so it's a 448 or a 486. I can't check the watermark. I know it's not really necessary to know with THIS stamp, but just for my own edification is there a visual way to tell them apart? Based on 1847usa there could be color differences, but this one looks straight green to me. Since it's a 1921 postmark, it seems more likely that it's a 486.

Date on the cancel could be of help on I.D.'s or even a light table, but for a postcard I'm not exactly sure what you can do? I know that you don't need to know on this particular one though.

Thanks ILS. I really don't need to know, just curious.

Possible answers can be found HERE:
http://en.wikipedia.org/wiki/Worces...ter_Railroad

Probably #486. If you want to be sure, you can lift it from the card, check the watermark and hinge it back in place.