If you're mainly going to be using them for pull-ups, you could look at the Metolius Power Grips instead (the oddly-shaped blobby wooden ones) -- I did a quick Google, and they seem to be selling for about 30 Euros on some German sites.
I haven't tried them, so can't vouch for them, but it looks like they could be interesting. Picture of them in use here:
FWIW, I've got fairly small hands, and don't find the Rock Rings that much use for finger strength training, because the pockets are so big for me -- the smallest one is a comfy three-finger pocket that goes past the first joint of my fingers.
So if I was building my training set-up from scratch, I might go for something different.
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I haven't tried them, so can't vouch for them, but it looks like they could be interesting. Picture of them in use here:
http://danblewett.com/2011/10/my-5-favorite-grip-strength-tools/
There also seem to be various wooden versions of the rock rings being made, e.g.
http://crusherholds.co.uk/fingerboards/crusher-orb-fingerboard
FWIW, I've got fairly small hands, and don't find the Rock Rings that much use for finger strength training, because the pockets are so big for me -- the smallest one is a comfy three-finger pocket that goes past the first joint of my fingers.
So if I was building my training set-up from scratch, I might go for something different.