Small printed name on my 'Strike Ruler Mk 4' is Blundell Harling Limited, see link below;
Drawing Boards from Blundell Harling
Ref number on the cardboard cover is 6605-99-110-5768. However, the ref number on the ruler is 1680-99-6379347
I also have a 'plotter, combat mission, plu-6/c'. The NSN is 6605-00-766-6901. This is a USAF version of the Strike Ruler made by Aero Products Research Inc, Mfr's Part No FAA-215. If you Google the NSN there are other manufacturers listed.
Hope that helps
G&J