Knowing that the leading expert on vintage airgun ammunition has to be John Atkins, I dug through past pellet articles of his in Airgunner Magazine,and in the December 2006 issue he mentions Lanes Bully Bullets. These are dated to 1906, and are not only one of the earliest mentions of waisted pellets, but they also have flat wadcutter head. It is interesting that when the waisted pellet was later accepted in Germany, the flat head type eventually became their norm, whereas in the UK, the flat head did not catch on, and waisted pellets became exclusively domed or pointed.
Frank's 1908 catalogue picture shows a domed head on the waisted pellet, and in my own searches I could only find German dome heads up to 1939 (catalogues seen from the years 1926, 1933 and 1934). The first definitive illustration of a flat head pellet I found was in a 1939 Akah catalogue, and these were made by Utendorfer. So it appears that the mystery pellets could have been pre-war, but only just.