Feltwad, when I saw your post, I Googled for "rifle with bayonet bar" and "German rifle with bayonet bar", and came up with a few examples. One was much like Taylor's post, a Model 1810 (New Pattern Jaeger) ca 1828 in the Royal Armoury at Leeds, War Gallery Object Number: XII.2282, and several German rifles 1750 and later with similar bars. I also found several bayonets that might match such a bar.
http://bayonetsonline.com/german-bayonets-for-sale.html has examples - an 1810/20 Hirschfänger bayonet, and several more, with slot in cast hilt.
At the same time, I wrote to friends across the Pond with lifetimes of museum and arms experience. I just heard back, as follows:
"The bar on the side of the muzzle of the rifle by William Landell was indeed for the fitting of a bayonet. The fact that the rifle was made in Scotland would suggest it was for deer stalking of course, but in Scotland at about the time this rifle was made, there was a strong interest in the volunteer or militia movement, what was later known as the Territorial Army; part-time soldiers whose primary role was for home defence in times of a foreign threat, leaving the professional army to go abroad as necessary.
There are many surviving examples of British military target rifles which were purchased privately by members of volunteer units in order to take part in inter-regiment and other military-style rifle shooting competitions which became very popular during the century. This sporting half-stocked rifle with an octagonal barrel 39 inches long would match the preferred length of the musket then in British military service, while being rather longer than usual for a sporting rifle.
Deer stalking was (and remains) popular in Scotland, and there was a greater chance that a member of a Scottish volunteer corps might have equipped himself with a rifle mostly for game shooting, but capable if necessary, of doubling as a military arm when fitted with a bayonet.
I’ve seen German jaegers with bayonet bars, intended for dual hunting/militia purposes. So, a suggestion of the owner being a militia officer is certainly a possibility. I know of a number of continental European military rifles in the Royal Armouries collections with bayonet bars of very much the form of that on your rifle. All of these take a proper sword bayonet with a solid cast-metal hilt (usually brass or a high-nickel alloy), with a groove from the pommel forward into the side of the grip whose profile matches that of the bar of course, and a spring catch to retain them in place,
The only other thing I can offer would be a sensible insurance policy on a dangerous game rifle for an intrepid explorer/hunter type… perhaps boar on the continent, or big game in India?"
Hope this adds to your really special rifle! Now we know why it is muzzle heavy. Thanks again for sharing. Bob
