Perf gauge mislayed but perfs same as stamp on a FDC. Tudor crown watermark. Shiny gum presumably gum arabic. Condition marred by some toning spots on back.
Sorry for late reply, but posting delayed by family illness.
Jonathan

I recently purchased this stamp as "missing blue".
Grey black colour, mint hinged. Unlike reprints mentioned in SG specialised vol 3 this has gum and no inscription on back. Watermarked.

What is it?