Firing on Leach standard ware tankard?

Good morning,
I have listed this under ID simply because I’m not sure what it is and once I know that will be a question I don’t need to ask again.

I think this is some kind of glaze issue burning in the firing (perhaps an impediment or tiny piece of wood) but I’ve never seen anything like it so thought I would ask!
The piece is a Leach standard ware tankard and has a clear SI mark under the handle curl.
I did see the ‘issue’ before I bought it but hoped it might be a makers mark but I’m fairly sure it’s not and would just ask your thoughts.

Possibly an impurity in the clay, like a bit of rust or iron-rich grit that's bubbled out and burnt on the surface. Or it's touched another pot in the kiln. Or it's a dirty fingerprint that's contaminated the glaze.

Thanks Naomi as always I’ve learned something so always worthwhile!