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extra hollow are the prima klang razors.
if you take a look at the wiki of this site there is the henckel's scale of hollowing - they had 14 different levels.

you will have to send your razor to...

welcome to srp

the usual recommendation is 5/8 to 6/8 hollow ground. most people find extra hollow trickier to use when starting, but the regular dovo grind is on the stiff side of hollows.

and...