Depends on whether or not the convertible is a REAL "old model".
OK. "Old Model" in Ruger-ese means "pre-1973".
357Magnum Ruger SAs prior to '73 came on a mid-size frame and left the factory with no transfer bar safety. A few of that series shipped as factory 357/9mm convertibles and it wouldn't surprise me if there was some collector value there. As I'm not a collector, I couldn't say for sure.
In 1973 Ruger revised the entire product line to include transfer-bar safeties, and stopped making mid-frame-size guns. All centerfire Ruger SAs of 357Mag or up horsepower were built on a 44Magnum-class frame, until 2005 when the mid-frame finally came back with the New Vaquero and 50th Anniversary Blackhawk in 357Mag.
Because all Vaqueros are built on the post-1973 action, ALL are considered "New Model" types. Ruger screwed everybody up calling the "New Vaquero" what they did, and that made people say "old model Vaquero" when it's a "new model" action.
Yeah, the whole thing is an Exedrine[tm] headache.
Anyways.
The older, large-frame Vaqueros are properly called plain ol' "Vaquero". Or we'll sometimes say "Old Vaquero". But not "Old Model Vaquero", unless somebody took a pre-'73 Ruger and seriously bastardized it to look like a Vaquero (sigh).