Greece--Inverted Watermarks

I recently purchased a collection of Mint Greece stamps from 1961-1979. These were all mounted on Scott International pages. The previous collector had inserted blank pages on which he had mounted many of the stamps with the heading "Inverted Watermark"

Scott does not list these "Inverted Watermarks". Are there any other catalogs, such as Gibbons, Michel, etc. that do list these? Is there any significant additional value for these Inverted Watermarks?

There's nothing about inverted watermarks in Michel and Greek Hellas catalogue.

SCB--Thanks for the reply. I am surprised that the Greek Hellas catalog doe not list them. I checked a few of the stamps from the "Regular" pages and the "Inverted" pages and the watermarks are in fact inverted. Well, since none of the catalogs list them, I would guess they are common and not generally considered collectible as a separate group. I'll put them in my duplicates pile for sale or trade.

If they weren't taking up much space, I'd keep them. Maybe someone has the Vlastos catalogue and can check for you...?