GTs58Schwinn frames were not built on the serial stamping dates. Good example of this is the MR serial stamped head tubes that Schwinn stamped out in December 1980 when the workers were on strike. No bikes were produced for 13 weeks, but parts were being made during that time. The December 1980 serial stamped head tubes were used throughout the 1981 production and some in 1982. They were also found on just frames that were sold in 1981 and on some early Sidewinders, more specifically, the ones that were painted White.
The serial number BS367823 on @Les frame indicates the bike was made for Schwinn due to the S being in the serial. Early Japan made frames for Schwinn had the letter S in the serial numbers and the placement of the S was at the beginning of the serial number on the early lightweights. It's been written that the Sierra and High Sierra were produced by Giant. The placement of the S may or may not indicate who made the frame, Panasonic, Bridgestone, Giant, it's only an indication that the bike was built for Schwinn and it has nothing to do with the dating of the frame. Les's frame has a serial number that was machine stamped on the BB shell before that shell was used to build a frame, just as Schwinn did starting back in 1937. The serial BS367823 was stamped in February 1983. When the frame was made or when the bike was built in anyone's guess.
The explanation of the Schwinn Chicago strike and production was not happening during this period I understand. There was some production going on I have seen but not necessarily of frames. These MTB frames where imported by the Schwinn Parts Division as per Jim Hurds comments of MTB in 1981. There appears to be confusion with the serial numbers of the BS367823 being a format that considers the S to indicate Japanese made which some of these frames are stickered as separately. You must understand the S as part of the Schwinn Chicago Serial number format definitely means 1981 and many of the frames using it exist for the entire year of all models, BMX and ATB included. S as part of the Schwinn Letour production made in Japan or others would mean being made for Schwinn but is based on the S should have been the first letter not the second if at all.
These are 2 different legal Serial Number formats and if BS367823 was placed on the frame in 1981 and for the same number to be used again in 1983 would be a breach of Serial Number legality. In 1983 I have read Schwinn Letour were being made in either the Chicago facility before it closed or at Mississippi, USA not Japan. From what I have seen on this page there is confusion not only in my view but others as well and could the frames had been imported with no number at and the BS367823 added by the Schwinn Parts Division who James Hurd stated imported the frames. James Hurd as being the curator of the Schwinn History Center could have had more in house knowledge as what was happening.
Les