This is a 1964 Greeves model 24 MX1, Challenger.  This model is significant for a number of reasons,  primarily the all Greeves engine.  Prior to this model Greeves purchased complete engines from other manufactures (mostly Villiers) and then extensively modified them (stripping and replacing just about everything but the cases)  Starting with the MX1, Greeves cast their own cases in its foundry and assembled their own complete engines.

The cases were so strong that they were used as a stressed member.  Unfortunately, the MX1's Greeves specification Albion gearbox was not robust and has a reputation for failing when used aggressively.  Another interesting feature is the leading link front suspension which uses torsion rubber dampers ("doughnuts") along with small Girling shock absorbers - simple and effective.

 
Technical Specifications
 

Model	1964 24 MX1 Challenger
Engine	246 cc 2 Stroke
Weight	215 lb
Gearbox	4 Speed Albion