Chasa has it 100% correct here.
I had one that I swore was a 491 for years, and when I finally sent it to Bill Weiss for a certificate, I told him I'd eat the stamp if he found a watermark. I never ate it, but he found a watermark.
Fortunate for me, I found, for $41 on
ebay about a year ago, a 491, with beautiful margins, scissor cut on one side, now with a cert. Would grade VF-XF easily, and I haven't seen one on
ebay as good as my copy since then.....again....$41.....$700 CV stamp.
Really, Really tough issue........good luck!
Ray