I recently saw a picture of a 7-key chanter on Peter Hunter’s site.
What are the extra three keys for?
Doc
Some classical sets had 6 to 8 keys on the chanter. There is no “official” number of keys, nor any set determination of which notes the keys should be for. Like everything else UP, it is open to wide interpretation. Personally, I consider the term “fully keyed” to mean 6 to 8 keys, not including the stop key. Modern pipemakers are throwing the term “fully keyed” around to mean 2-, 3- and 4- keyed chanters. YMMV
You do not, by rights, need any keys on a UP chanter. The extra notes can usually be achieved by half-holing or cross-fingering. To play most ITM, it is “convenient” to have a Cnat and Fnat key. These are more useful in the 2nd octave, but are also handy when more speed and accuracy are desired, versus the extended tonality that can be achieved through half-holing or cross-fingering (or combination thereof). The Fnat can be a ring-type key or a long key. In the Hunter example, it is the lower, ring-type key.
To make the chanter more achromatic (or is it chromatic? - I can never remember) it is also handy to have keys for G# and A#. Like the Cnat and Fnat keys, they are handy for when speed and accuracy are desired. The Hunter chanter has both.
Some people take exception to this, but you can also have a key for the back d. It is not really useful until you need to hit 3rd octave (d’). Some reed/chanter combinations can’t hit this well on their own, and the key just helps things along. The Hunter chanter has one of these. It’s the long one that wraps around to the front.
There is almost no use for them, but classical chanters would also provide keys for eb’, e’, and/or f’. Whuffo? For 3rd octave again. The little one above back d is for one of those - can’t remember which off hand - I think it’s for e’.
Naturally, UPipers can only think in terms of the key of D. All the keys notes mentioned above would have to be re-interpreted for the key of Bmaj:
Eb => Cnat
Enat => C#
Fnat => Dnat
G# => Fnat
A# => Gnat
Cnat => Anat
Dnat = > Bnat
djm
I suspect that some of the “gentlemen pipers” had the means to pay for all sorts of optional extras on their pipes, even if the usefulness or practicality of such extras was dubious. 3rd octave keys is an example (other than D, I’ve never heard a 3rd octave note being payed). Double bass regulators is another (never heard one being played).
Thanks guys.
Doc
I recently acquired one of these chanters from David and Benedict… lovely things… I love all the hardware! 
David told me there is a historic precedence for the 7 keyed chanter, but I forgot what it was- Coyne, Kenna, Taylor?
All of the above… Kenna and Coyne often included keys for third-octave-e, second-octave-c#/third-octave-d (same key), and Eflat, in addition to the ‘usual’ Fnat, G#, Bflat, Cnat. In fact G# was rarer than d3 and/or bflat.
There’s a Taylor chanter with 8 keys - all of the above plus a higher key that seems to be third octave F#.
Oh yeah, Egan included such keys too. Their omission seems to be more of a 20th century trend, at least after they became common in the early 19th century.
I’ve seen one chanter with blocks for 9 keys (5 of them “ring”-style keys!) but it seems to be a bit of an anomaly.
Bill
Or a threat. 
djm
Quote “I suspect that some of the “gentlemen pipers” had the means to pay for all sorts of optional extras on their pipes”.
Many years ago I visited a fellow Northumbrian piper at his place of work…the music department at Sotherbys auctioneers in London. He had a photo album of pipes that had been through there which included an over the top set of uilleann pipes which he suspected had been made as a presentation item. It had six drones…the usual three in D plus a second set in G, and had semi precious stones inlaid on the chanter between the holes!
Ian