By Stanley of London. Stores reference 9/421. Alloy 'ruler' approximately 53 x 5 x 0.5cm. One edge is marked in inches and tenths of inches. The other marked for 'Speed In Miles Per Hour (from zero to 150) and 'Bombing Error In Yards' (from zero to 500). A further scale 'Speed In Miles Per Hour Hill's Mirror Scale' is marked to the centre of the rule (zero to 150). The rule is scratched and has some fairly minor edge damage/nocks. Clear King's crown Air Ministry marks. Unusual and rare item. The first we've seen in over 30 years. Stock code M20455.