What kind of Machin is this?

That is just part of going through the postal system there will be other dots on the envelope, believe it is something to do with the postcode

It's a tagging dot from the sorting machine. You will see it on GB stamps (mostly definitives), and occasionally on other British Commonwealth stamps. I should have a pretty old post on it somewhere on Stamp Bears. I'll see if I can find it...

Well, I guess the post was on a forum far away that has since been wiped. But they are sprayed on by the sorting machine. I've seen squares, "diamonds" (maybe an orientation issue with the sprayer), and circles. All the ones I've seen are generally that "color" (grayish, silverish, or darker). Sometimes it gets applied onto the stamp, but as far as I know, that is not intentional. So you see it on some GB used stamps but not most.

I think the most I've ever seen was 3 on a stamp. Maybe I.M. DeBear has seen more in his collection?

Yup, circles, squares, & diamonds. There's also a pale blue, and a reddish orange.

GB-USA postcard 05APR1994, phosphor dots on tied Airmail label