Worthy Of Sending For Cert?

A starting point would be to buy a perforation gauge, or print a reliable one. I think the Stampsmarter site has one you can print.

An alternative is to look for stamps that can only have the relevant perforation gauges and see which closest matches your stamp's perforations. That may help decide whether your stamp can be the extremely rare one that OP thinks she has.

You have the same, stamp? If perforated it is not a 613.

As to where you should be posting, use the inherited Stamp section to which I directed you on the second thread you started in "Introduce Yourself."

My 2c black Harding, perf 11 w/ no ink specks on the back… measures 19mm x 22mm, thus it is a flat plate printing stamp that must have been on the 1st sheet of the day. I wanted to follow up on how it turned out. Peace out and happy stamping!

I may have missed something here. From what I understand, ink spots occur on the flat plate printing when printed sheets are stacked when the ink is not yet completely dry. If the ink has dried, there would be no reason to expect the ink spots on the back of even the flat plate printings.

So, must it be the first sheet in a stack that has no ink spots and was there just one stack on a day?

Or can any sheet from a flat plate printing lack these spots if the ink from the previous sheet had dried sufficiently? Could even the later sheets in a stack lack the spots because there is little pressure on those?

The four plate power presses operated surprisingly fast (roughly 15 revolutions per minute). There were only a few seconds for the lady to remove a freshly printed sheet and place it onto a stack. No chance the previous sheet was completely dry. As far as I remember, after every one hundred sheets (I guesss it is 100), a blanket was put onto the stack. So every hundredth sheet likely had no setoff on the back. I think a stack was several hundred sheets until it got shoveled away and a new stack was started. Many stacks per day were produced on a 4ppp.