Found A #557c Help To Identify?

No, spacing is somewhat irregular. Partially punched perforations are sometimes seen but are not definitive.

Looking at the comparison view, the second and third perf tips from the left are uneven with an unexplained piece missing from the tip of the third perf, possibility part of an original perf hole that survived alteration.

I agree that the bottom perfs look suspect. Having said that, having seen genuine, certed, perf 10's on one side, they also look odd to me, often. On the stamp in question, it appears that the holes were created with some kind of uneven pressure (for lack of a better way to explain) around each perf hole. The left-most hole doesn't appear to be in line with the rest of the bottom holes. Neither of these absolutely disqualify your stamp, though.

You will never be able to sell this as a 557c without a cert. If it is, it is worth many $1,000's. If it isn't, it is worth pennies. Do you want to roll the dice, with $50-ish on the table?? Only you know that. If it were me, with THIS stamp, there are days when I would, and there are days when I wouldn't bother. In the end, I guess it would get sent in for a certificate, only because it only takes one day when I would roll the dice, even if there is a month of the other days.