Here's an illustration from an article about instras by The Quack Doctor:
The Instra was patented in 1896, and I found a couple of newspaper ads for them from 1897. The first appeared in February 1897:
(Patented throughout the world.)“WARMTH IS LIFE”The POCKET INSTRA is imperceptible in a pocket, ornamental, and by the special patent in its construction, ABSOLUTELY SAFE and CLEANLY. The Instra prevents chills, is invaluable to delicate persons, who by its use can resist cold, changes of temperature, or draughts. Useful when driving, shooting, bicycling, travelling, or at theatres, public meetings, churches. PERSONS USING THE INSTRA CAN SIT IN A COLD ROOM IN COMFORT WITHOUT A FIRE.
D. BLAIR & CO.. 47. Cannon-street, London. E.C.
And in September 1897, when the Klondike Gold Rush was at its height, the Instra was advertised as the ideal equipment to take to cope with the cold of Alaska - most of the prospectors had to carry their own equipment, so the weight of an Instra was essential information:
KLONDIKE GOLDFIELDS.—Those intending to go out are recommended to take a POCKET INSTRA WARMER, which weighs only 3½ ounces. With this in their pocket they can defy the cold. Innumerable testimonials from all parts of the world.
The "Colonist," August, 1897, says:—"An Instra and refills for 12 hours' warmth weighs only four ounces; 100 refills, producing about 300 or 400 hours warmth, weigh only about three-quarters of a pound. While everybody is desirous of finding gold in the Klondike Goldfields, all the world seems to be afraid of the cold. The Instra would seem to be the solution of the terrible climatic Klondike problem."
They must have become popular in some quarters, so that in 1914, they could be referred to without the capital i, expecting that readers would know what they were. It could have been a huge marketing opportunity for the company, but evidently most soldiers on the Western Front made do with mittens - it was only Indian troops (so from a much warmer climate) on outpost duty (so especially exposed to the cold) who merited instras.