High Frequency Laboratories Mastertone 1931 Get a print! $8.00 Add to Cart Product Information: Model:Mastertone 1931 Manufacturer:High Frequency Laboratories Schematic Pages Page 1 Schematics Content Page 1: HIGH FREQUENCY LAB. Model 1931 Mastertone Please note: we carry a wide range of products which will fulfill the specifications above. Please use our search function or browse our products to find what you need!