K&E 4053-3T, 10" (magnifier *1.6)
Keuffel & Esser 4053-3F, 10" slide rule. This is quite a scarce slide rule - the F stands for "Fine" because it has finely divided scales. Finely divided means that the scales are subdivided as closely as the 20-inch slide rules. Combined with the included magnifier on the cursor, this means greater accurancy when reading the scale - it is almost as accurate as the K&E4053-5 (20") slide rule, but more compact.
This is one of a series of similar K&E slide rules in different lengths - K&E4053-2 (8"), K&E4053-3 (10"), K&E4053-3F (10" but with magnifier and finer scale divisions) and K&E4053-5 (14").
D Scale length: 400 mm. Significant digits: 3.3 to 4.3.