Proportions/type-setting

Data:

• 18-inch proportions/type-setting rule.
• Scales: A = B P L C = P
  The P scale is folded at 6, which might indicate inch to pica conversions.
  The L scale is folded at 14, which might indicate lines per inch. Mike Syphers
• Have seen only one other for sale.
• Somewhat similar in construction to American Blueprint Military rule.

Questions:

1. Who was the maker?
2. What year? (perhaps 1940/50s)