In a later article, vol. 5 no. 2, he added an earlier type, type D, said to be from about 1921; and a type E, built into a walking cane.
The differences between the types are subtle. See the comments by the picture below.
Type D: Oldest versions, not shown. Type D is like type A but with a squared off lower edge on cursor, and a triangular-profile top cap, and possibly other variations. These are rare; several model N units are known, probably made around 1921.
Type A: Serial number 7450 (left) is a good example of a type A. Made around 1921-1923, without letters on the serial number or with the letter "A," these early units usually had a key-chain loop installed in the top cap, or knob. The cursor is a single lightweight metal tube, usually plated (nickel, probably) but sometimes found in brass, with index marks in the form of arrows. These rules are marked with the Holborn Viaduct address on the scales, but do not say "Made in England" anywhere.
Type B: Serial number D200 (center) was purchased new by Roy Ewans when he started college in 1935. The defining characteristic of this type is the screw that holds the two-part cursor together (actually, there are two screws--one is around back). The type B is known for quite a variety of different top cap (knob) shapes--usually they have a notch to help locate the ONE on the top scale, but this old one does not. The top cap is marked "MADE IN ENGLAND" on most, as on this one.
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).
Type B/C hybrid: Serial number R3474 (right) seems to be a transitional design on the way to type C. The two-part cursor is held together by a dimple (the small white dot by the lower index mark) in type C as in this unit. But type C as originally defined has "MADE IN ENGLAND" on the handle bottom, around the serial number, as opposed to on the knob as in type B. Otherwise, this unit looks like a type C.
Type C: I traded away my type C Model L serial number Y1115 before I thought to scan its top and bottom ends, but you can still admire its scales plain or unrolled.
Type C units are common for serial numbers starting with R, S, T, U, V, W, X, Y, Z, A, and B.
This type B/C hybrid is a Model K, with scale numbers 414 and 423.
All of these units have black-on-white scales. White-on-black scales are also found, apparently mostly among earlier units. The varnished paper scale on the well-used type B has yellowed significantly, while the scales on the older and newer units have stayed white, probably owing to disuse and careful storage.
The two older units shown are nickel plated. The shinier unit on the right, like most of the Otis King's rules found, is chrome plated.
We seek information on additional variations, more confirming examples of the patterns found, etc. See the request for info at the bottom of the Top Otis King's page