Speed model with Semi-Sports body must at least be a rare combination. Especially as the Speed Model had a slightly different chassis which might cause a redesign of the body to fit. More probable that a speed model engine later was mounted in the car.

Arnold Davey in his book "2, 3 & 3 1/2 Litre in Detail" explains the development of the speed model on pages 46-63. It was presented first at the Brooklands fete in August 1927 (btw. nice photos from the Brooklands fete to be found here: http://www.austinharris.co.uk/event/lagonda-fete)
Arnold describes the changes to the engine (e.g. changes of oil supply) to make it capable of higher revs and chassis (moving the engine back by 150mm and the radiator directly above the front axle, different chassis side and cross members. These differences should make it possible to decide whether your car is a speed model or a 14/60 with speed model engine.
Armold also gives the following differences in technical data of the engine: compression 6.8 instead of 5.9, oil capacity 3 instead of 2 gallons, different valve timing (more overlap)