Some info that I found
RGB1507-1508 series
A range of gearboxes primarily for driving high reving engine, rev counters (tachometers) and designed to run at speeds of up to 7000PRM when fitted with ball race bearings, and up to 4000 RPM with self lubricating bronze bearings. Various ratios are available as well as reversing and non reversing.
RGB1507 (Ball Race) Maximum Input Revs 7000RGB1507- 00 2:1 Ratio Non Reversing
RGB1507- 01 2:1 Ratio Non Reversing Long Spindle (.890”)
RGB1507- 02 2:1 Ratio Reversing
RGB1507- 03 4:1 Ratio Reversing
RGB1507- 04 2:1 Ratio Reversing (output .120” Internal Square)
RGB1507- 05 Not Allocated
RGB1507- 06 8:1 Ratio Reversing
RGB1507- 07 8:1 Ratio Non Reversing
RGB1507- 08 1:1 Ratio Reversing
RGB1507- 09 1:1 Ratio Non Reversing
RGB1507- 10 1:1 Ratio Non Reversing Long Spindle
RGB1507- 11 2:1 Ratio Reversing Long Spindle
RGB1508 Maximum Input Revs 4000RGB1508- 00 1:1 Ratio Painted Black
RGB1508- 01 2:1 Ratio Reversing
RGB1508- 02 1:1 Ratio Reversing
RGB1508- 03 4:1 Ratio Reversing
RGB1508- 04 1:1 Ratio Reversing Painted Grey
RGB1508- 05 1:1 Ratio Reversing
RGB1508- 06 2:1 Ratio Reversing Painted Grey
RGB1508- 07 2:1 Ratio Non Reversing
RGB1508- 08 1.5:1 Ratio Reversing
RGB1508- 09 4:1 Ratio Non Reversing
RGB1508- 10 3:1 Ratio Non Reversing