Among the Lewis models featured in the 1909 Lewis Motors catalogue was a water-cooled motorcycle with a ROC two-speed transmission. This two-speed conversion could be fitted to most machines, if somewhat clumsily as the conversion required replacing the entire back half of the motorcycle frame. For some machines this was as simple as unbolting the original rear end (one bolt at the rear of the engine plates, another at the seat lug) and bolting on the ROC conversion, but for the one-piece Chater Lea frame as used by Lewis some serious rebuilding was required.
By comparison, some of ROC's competitors (such as the NSU two-speed epicyclic pulley) could be fitted with little or no modification to the machine.
ROC motorcycles, and the ROC two-speed conversion, were products of A.W. Wall Ltd., Birmingham. From their period advertising, we see that their focus was on their rather eccentric long wheel base motorcycles, and their two-speed conversions. Typical is this advertisement from The Motor Cycle, 17th February 1909:
The similarity of the transmission to that of the 1909 Lewis-ROC is clear, but there is more to be learned from looking closely at the motor. Comparison with the 1909 3 1/2 h.p. Lewis shows that the motors are clearly from the same source, sharing the unusual features of the two-bolt exhaust mounting, circular timing chest with magneto drive from the inlet cam, and the unusual tappet guides.
As used by ROC, the motor has "ROC" cast on the timing side, and "ROC A.W. Wall, Birmingham" on the drive side, but the Lewis version of the motor bears no maker's mark anywhere, save for the usual script "The Lewis" on the magneto chain cover. It is not known whether Wall built their own motors, but it seems likely that they did, and that they sold an unbranded version of it to Lewis in 1909.
The 5-6 h.p. twin mentioned in the 1909 advertisement above also provides piece in the Lewis puzzle. When we see it illustrated (as in this advertisement from Motor Cycling 31st January 1910) we see the same motor that powered the 1909 Lewis 5-6 h.p. twin.
Ideally we would have access to letters, ledgers and business records for Wall and Lewis, and we would be able to analyse the business relationship in detail. But we are not that lucky, and we'll have to make do with what sparse information we do have. Guessing, based on period reports, photographs and surviving parts, I'd say that the 3 1/2 h.p. "ROC" model was a minor part of Lewis output, with certainly fewer than 100 machines built between 1909 and early 1911. On the basis that the 1909 Lewis Motors catalogue provides the only evidence of the existence of the ROC two-speed water-cooled model and the ROC-powered 5-6 h.p. twin, we'd have to say that these were produced in very small numbers indeed.