But, the question is whether there is a premium for engraving by Harry Kell and, if so, how much is it?
Beautiful gun Ted, and looks very Kell to me.
Yes, there is a premium for Kell engraving and it has been that way for a long time. Kell produced several different types of scroll work from his shop, not only what we know as "Fine English Scroll" but also other more robust scroll work such as this photo show and I might add the work is pristine:
https://www.john-dickson.com/available-gun/john-dickson-son-sidelocks-made-1919Whether the premium is 10 per cent or 20 percent is is not consistent as the premium is directly proportionate to the particular gunmaker and also to specific gunmaker's work that is in vogue. Today for example such a gun in Vogue is a John Dickson & Son round action gun with 29 inch damascus barrels (of let's say .026" minimum wall thickness) that was made in the late 1880's about the time that Dickson moved the position of the cocking indicators is one of the guns most highly desired among wingshooters. Viewing several of the Dickson round actions with the said specifications both Kell engraved and not ---all of the same or similar condition will provide you with what the seller believes the "KELL" premium is valued.
In the case of a 29 inch damascus barreled John Dickson & Son round action gun that I have chose to reference, it is not just that the gun is in VOGUE that make is so desirable today, it is that astute gun enthusiast have realized that it is the best wingshooter's gun ever made. Some prefer a bit shorter barrels for grouse and quail shooting in close quarters, but that is OK. In either case you will see the Kell premium illustrated.
It is maybe worth saying that in many Kell engraved guns all the work was not done by the master himself; and those of you who are educated or experienced in engraving techniques and styles will be able to examine a Kell engraved gun and see that there is difference in such areas as the borders and trigger guard that do not show the same skill or techniques that the main body of engraving. This is not just on Kell engraving but on all engraving of the time as the engraving "houses" had apprentice and sub master engraving at work learning to be masters later. Ken Hunt was an apprentice under Kell.
Now to discuss the intangible value difference in premium within the Kell styles of engraving. This intangible value is in the eyes and the art taste of the beholder. I personally like Fine English Scroll of all engraving styles and as Marcus Hunt has shown us not all engravers know the rules and techniques of how to engrave Fine English Scroll, but Kell did even though he did not invent Fine English Scroll. Therefore If two guns of the same quality and maker were on the table before me with one of Fine English Scroll and the other of the scroll style illustrated in the photo I would pick the Fine English Scroll even If it cost more. It is note worthy that I have never seen and Italian engraver who could execute Fine English Scroll properly ---I suspect there is one or two it is that I have not seen it. One of the other intangibles is Kell animal engraving. He brought a whole new illustrative approach to engraving animals for his time. His animal engraving is no where as good as say Simon Coggan's Cape Buffaloes but for it's day it was several steps above others.
In America today this engraver is a standout;look at his animals:
https://www.brianhochstrat.com/Stephen Howell