The Foursome 1.2 cc diesel was a somewhat agricultural-looking but very well-made British sideport diesel produced in relatively small numbers during the years 1948-1949 by a long forgotten manufacturer going by the name of Seymour, Hylda & Co. of Southwick, near Brighton in Sussex. Almost nothing is known about this company apart from the fact that they were a firm of engineer's instrument makers. The quality of construction of their products is certainly well up to toolmaker standards.
The engines were distributed exclusively through the well-known model shop and mail order house of Arthur Mullet & Co. of 16 Meeting House Lane in Brighton. Oddly enough, Mullet marketed the engine under the name Foursome Products, although the address of this entity was that of Mullet's model shop.
The Foursome was noteworthy for its very unusual construction as well as for its extremely long stroke in relation to the bore. The latter feature gave rise to excessive levels of vibration, although the engine started and ran very well within that limitation. However, it was no match for the competing Mills 1.3 model, hence fading from the scene relatively quickly. Examples are very rare today.
Full details of the Foursome and its Kalper .32 cc diesel companion from the same manufacturer may be found in my earlier Kalper/Foursome article on Ron Chernich's "Model Engine News" (MEN) web-site.