The stem referenced in the wooljersey image is a ttt. The bars of that period were bulge formed at the center, original were stamped Masi. About the widest will be 42cm.
Late Cinelli bars will fit as they too are 26.0mm. Can be found up to 44cm for sure, perhaps wider. The chances are good that a Cinelli bar of 44cm or more will have the smaller clamp diameter, many folk cannot measure the .4mm difference accurately if not purchasing in person.
If looking on ebay, 3ttt is often used as a title, rather than ttt.
John Jorgensen
Palos Verdes Ca