The first US made intermediate line I remember (in the mid 1960's) was made by SA for O***s, it was a woven dacron with amber coating (to look like the old silk lines). It was made in WF & DT from 3wt to 12wt and was very popular for both fresh and saltwater FF.
In the 1970's SA had their own Intermediate lines in a light green color, both DT and WF. Lefty showed us how to take a 8wt or 9wt DT and cut it in half to make a 41 ft head, we would direct splice an inter. shooting/running line to the head and make up our own "Long Belly" Intermediate lines. This was well prior to the commercial longer head WF lines.
Cortland had an early Intermediate in WF and DT in a bright blue color, it was similar to the SA/O***s line and many liked it for fresh water streamers and wet flys.
Later SA introduced the Monocore lines (for saltwater) which were amber colored (later clear) mono WF lines,,,,they never became popular due to the short 30ft head length. These were the original "Slime Line".
A few years after the SA Mastery series was created they introduced the Stillwater lines for fresh water lake fishing. In addition, SA made special tapers for saltwater with clear line variations.
Regards,
FK