Type B units are typically found for serial numbers starting with C, D, E, F, G, H, J, K, M, N, P, R, and a few S and T; and maybe some early A and B. These type A and type B rules are Model L. The type A early Model L has scales number 416 and 417, while the type B, like most Model L’s, has scales 429 and 430 (the upper and lower scales each have their own number printed on them). Otis King Slide Rule Classification Types Additionally, an otis kings can also be differentiated from its scales. Model  K : 414/423, Model L : 429/430 & 416/417 on the old ones, and the rarest Model N 424/422 

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1960: Otis King Rotary Slide Rule - abt 1960 - Type L C Serial T6205, Scales 429 and 430¹

Type B units are typically found for serial numbers starting with C, D, E, F, G, H, J, K, M, N, P, R, and a few S and T; and maybe some early A and B. These type A and type B rules are Model L. The type A early Model L has scales number 416 and 417, while the type B, like most Model L’s, has scales 429 and 430 (the upper and lower scales each have their own number printed on them). Otis King Slide Rule Classification Types Additionally, an otis kings can also be differentiated from its scales.Model  K : 414/423, Model L : 429/430 & 416/417 on the old ones, and the rarest Model N 424/422 

¹ see also eBay.co.uk Guides - A guide to collect and value an otis kings calculator (↑)