There were a number of types produced
1) Terry were a manufacturer of accessories of all types.
There is a copy of a Terry catalogue at Terry Catalogue This catalogue includes the following two items
a) Terry Magneto spanner BA series
BA is short for British Association screw threads which were small screw threads. Wikipedia has a good description see link BA threads
"Terry"s RgNo673876 Spanners"
the smallest being 1, 2, 3, 4, 5, the largest.
The 6, 7, 8, are marked on the reverse side to the 1-5
The larger three , 6 (smallest of the three), 7 and 8 (largest)
b) Terry Tappet spanner
This one is 4BA & 6BA
2) H.T.& S magneto spanners
This has the army arrow and 1944
3) BTH Magneto spanner