From Which Plate Is This Penny Red ?

I agree that it could be a plate 9. I, also, agree that it may be impossible to be sure.

That Penny Red from plate 9 example above, I doubt is a plate 9 example. If it is, OP's stamp, certainly, is not.

You have to use the two sites I've recently linked to CS. One has the sheets and the other has the missing stamps from those sheets.

You also need to read the inks that Rogdcam provided you. If you read the first Warwick and Warwick it says only 7 of the 11 penny black plates were used to print the penny reds. With that clue it is easy to google which plates. First link I found which had this info was: http://gbimperf.org/html/1d_black.html. It states that plates 1b, 2, 5, 8, 9, 10 and 11 were used. Based on this info you can then use the site that Bobby De La Rue is providing you.

Only 6 of 10 were used. Plate 11 is not a Penny Black plate, but a Penny Red plate that was used for an emergency printing of Penny Blacks.

1b is not a plate, but a state of plate 1.

Oh thanks for all these informations everyone ! That really helps !

Fisher-Brown requires the right corner frame.

The E-gauge - irrelevant as OP's stamp does not have a right corner frame - is accurate with a minimum of 800dpi scans. It also requires that if the scanner scans at 800dpi or better, it does not do so by interpolation.

Yes, and you know that because you read the instructions. The point is that tools are given and then you figure out what tools you want to use in what situation. There have been all kinds of links given to all kinds of info and frankly the OP could have found the same info with a simple Google search. We all are here to help newbies. At some point the work needs to be done though.

That is why I sent my message to illustrate to demonstrate how easy it was to follow the links provided and of course with additional assistance from Google..