Errrrm, at the risk of heading mildly off topic..... can anyone post a picture of a more antique economical FPL setup.
I am intrigued.
My grandfather, before heading into telecoms, was a fully trained mechanical S&T apprentice, joining in '36. even today when given scheme queeries, economical FPLs are brought up as an example of how to.
Perhaps one of us with a more historical bias might give a synopsis of the locking bar versus the fpl and the economical FPL, and the regs governing usage and installation.