I don’t know the answer how long these regulators last, but there was a slightly related thread not too long ago with a bit of useful information. At some point in the kind of recent past, Miller welding bought out Smith. The picture that you show is of the more modern Miller-era Smith design.
Personally, I haven’t had any problems with the current metal adjustment valve design. As far as I know there’s no necessary adjustment or maintenance of the regulator. Well except to keep it clean and dry.
Maybe someone else knows more?
Jeff