I will scan some drawings and photographs later today. To be going on with here is some text that I wrote for Rick Landers*:
"In May 1916, a long-range type of grenade designed by Charles Sangster was submitted to the Munitions Design Committee for consideration. The grenade had a pear shaped, serrated steel body with a propeller type wind vane on the front end and a 16 inch coppered rod. A central chamber housed the cap and detonator, which were inserted by removing the wind vane and its socket. The striker was fixed to the external wind vane which was supported in a threaded socket at the front end of the detonator holder.
Tests were carried out at the Clapham and Wembley Experimental Grounds in late September 1916. Trials were conducted for range, accuracy, safety and detonation and rough usage. The flight of the grenade was reported as very steady with a mean range of 363.4 yards at an elevation of 40. It was safe to handle even when dropped from a height of 8 feet with the safety pin withdrawn. Rough usage and dampness tests were also satisfactory with all test grenades detonating correctly. On 29 Sep 1916 the Munitions Design Committee recommended that approval be given for the adoption of the grenade.
The preparation of the grenade for firing involved removing the firing mechanism with a spanner and inserting the cap and detonator with the aid of a special padded rod. A safety pin secured the striker until it was removed just prior to firing. During flight air moving over the vanes turns them freeing the striker from the threaded end of the socket. On impact, the striker overcomes the creep spring (if fitted**) and strikes the cap, which ignites the detonator setting off the bursting charge.
Trials were carried out using both 43 grain and 46 grain cartridges to test the strength and fragmentation of the body. These tests proved unsatisfactory with 90% of the grenade being recovered in 16 pieces. A demand for 10 000 grenades was raised and further tests were carried out on 17 Nov 1916 with smooth cast iron bodies and 15 inch rods. Following these tests, the committee recommended on 8 Dec 1916, that 1000 of these grenades with cast iron body and 15 inch rod be ordered for troop trials. Despite the fact that the Sangster No. 25 grenade outperformed other grenades in similar trials, it was not adopted. Its official nomenclature was Grenade, .303-inch Rifle, No. 25, Marks I and II. "
* Grenade by Rick Landers 2002 (Jeremy Tenniswood has copies for sale)
** I have only seen a dozen of these grenades but not one had a creep spring fitted
Photo below shows the Mk II over the Mk I
