I've often wondered about this myself. "Subway" would be the last game produced using the cast metal end caps. However, many I've seen, including my brother's game which came out of an arcade in 1977, have the newer style bar. This leads me to believe that it's likely that much, if not all of the production run of "Subway" already had the newer style bar installed.
"Cross Town", which was one month earlier, is usually found with the older type bar.
"Dancing Lady" had many "firsts" for Gottlieb, and featured the new style lockbar on all copies produced.
"Hurdy Gurdy" should have the cast metal end caps, but it's possible that some were converted by operators, since the new bar was readily available as a spare part by the end of 1966.

My brother has a "Subway" which he got from an arcade back in 1977.
We played it in that arcade for probably two years prior to that, so it was only nine years old at the time.
It also has a very low serial number, but it had, and still has the later type lockdown bar. This leads me to believe that most, if not all copies of "Subway" were fitted with the new style bar, although it wasn't officially announced until the release of the subsequent model, "Dancing Lady".