Note
FT-301D
QST, Aug 1976, inside back cover
FT-301S
QST, Dec 1976, inside back cover.
FT-301SD
QST, Jul 1977, inside back cover.
The 301D was the first unit advertised (Sep 76), followed by the 301S (Dec 76). All four are featured in May 1977.
Base on the ad copy, the differences appear to be these:
FT-301 – 200 watts, analog readout (no ad details)
FT-301D - 200 watts, digital readout (QST, Aug 1976, inside back cover)
FT-301S - 20 watts, analog readout (QST, Aug 1977, p. inside back cover)
FT-301SD – 20 watts, digital readout (QST, Aug 1977, p. inside back cover)
Note that some third party ads refer to 40 watts PEP for the "S" versions. For example: QST, Aug 1977 p. 165, for Amateur Electronics Supply.
References
Brief description. Popular Electronics, Jan 1977, p. 12.