Now it looks like I’ve found the patent for that MM spray lance….
http://www.lens.org/lens/patent/GB_513013_A
513,013. Valve-actuating mechanism ; nozzles. MARVEN B., and MACPHERSON, A. J. March 29, 1938, No. 9618. [Classes 69 (iii) and 135] A hand-controlled nozzle of the kind in which a spring closed valve is unseated by a rod 15 moved axially by a hand lever 20 pivoted to the forward end of the nozzle body, is provided with a link 23 pivoted at 24 to the lever 20 and bearing on the rod 15, guided substantially along the axis thereof by a pin 21 working in a siot 22, the pivot 19 of the lever being in the line of the axis of the rod. The link 23 is normally at an angle to the rod 15 and so arranged that the toggle system constituted by the link and lever may be brought towards but not beyond the dead centre. An additional spring 25 may be provided bearing against the lever 20.