From before they got 'Fenderised', this is made by GrooveTubes.
This one states 'for acoustic and electric guitar (3.2 oz)'...but it can open up to over 20mm so I'd have thought that would fit most instruments within reason.
Excellent condition...still in the original packaging!
If you're wondering what the hell it is, here's Fender's blurb:https://shop.fender.com/en-GB/accessories/miscellaneous/fatfinger-guitar/0992180100.html?rl=en_US
Will post to mainland UK for £15
£12.50, payment by Paypal 'f&f'. SOLD!