The top one has ink bleeds indicating a flat printing... The bottom one doesn't. I see it sitting alongside a 575/577, which makes me doubt it. I am also told, the ink bleed is usually and indicator but not always... Am I right?
chasa ... when you say look at the back, are you thinking of the gum breakers? If gum breakers are present, I assume it must be 631. But if there are no gum breakers, then I would think a brief measurement might be called for in case it happens to be the less common 631 without the breakers.