This is an unashamed clone of the eighth variant of the Wen-Mac .049 Mk. II from America. It was supposedly manufactured by the well-established London firm of D. J. Allen Enginering, more famous for their Allen-Mercury (A-M) range of model diesels. However, there is overwhelming evidence to suggest that it was in fact merely assembled in England by Allen Engineering, primarily from parts produced under contract by Wen-Mac in America. Production was confined to the years 1959-1961, with relatively small numbers being manufactured in total.
The spring starter supplied with the original batch of these engines invariably failed after only 100 or so uses. The engine was soon released in a slimmed-down version without the starter, but the damage to its reputation had been done. It never achieved any sales prominence, hence being relatively rare today.
For a full discussion of this fascinating story, see my earlier article which is still to be found on the "Model Engine News" (MEN) web-site.