Three VT21 engines developed by Hirtenberger Patronen, Austria, are shown on a wooden display base. The prototype on the left was manually machined from aluminum and steel bar stock. It is signed by “Pesch” (its creator) on the side of the crankcase, and is believed to be the first HP VT engine built. It is physically smaller than the production engines bearing a bore of 14mm and a stroke of 13mm, making the total displacement only 20cc. Another notable difference is that it has an oil reservoir in the crankcase with a filler cap in back of the engine.
The two on the right are die cast production engines, each with a total displacement of 21cc, only 1cc larger than the prototype. The engine on the far right is a cutaway to exhibit the rotary valve and inner workings of the engine.