The frameset came with a Cinelli stoker stem, but I gave up on using it for two reasons: It was too short, so the rear handlebars would bump againt my thighs, and it was the obsolete 26.4 handlebar clamp diameter.

The stem shown is actually a garden-variety 1 1/8" threadless type single stem, with a shim around the seatpost to bring it up to the correct diameter.