Got Some Questions

Whilst taking a break from the thousands of washingtons I'm trying to sort ( it's going to be the end of me lol ) I've been sorting some others I've got earlyer stuff
And I am slightly confused but think I've got it right.... Grills going up and grills going down. Grills going down are stamped from the back and grills going up are stamped through the front of the stamp. I know it's a silly question but I just haven't been able to get my head round it.
And another question although it's back to the washingtons
518/518b is it common to have a impression on the back and does it carry a premium and does it have it's own no:/letter:
I think it's a 518b but undecided, but it is deffently brown not black violet and the impresion on the back is the same colour as a 395 but as I said I'm not completely convinced at the mo

You are correct with regard to the grills (what side of the stamp they protrude from).

As I understand it, the only way for a stamp to have an image on the underside is for the sheet to be laid on top of another with wet ink. If this is the case, this does happen from time to time, and does not bring a premium.

Thank you
I just couldn't get my head round the fact that it is the opposite way that a stamp is left
And I know some stamps have there own no for having a impression on both sides

And why is it that the more unusual stamps you find have hidden damage

You have asked why it is that "the more unusual stamps you find have hidden damage."

I suspect that most of those type of stamps were previously picked out of an accumulation or included in a stamp album from the past. Since the "more unusual" are typically the scarcer varieties, most collectors don't have a stock of that many to choose from to select the best for their album or collection, so they take extra effort to either repair a stamp or otherwise make it look presentable enough until a more sound copy can be obtained.