The Brian Bowdren article describes the SB-601 and SB-603:
SB-601: Although I have never seen an example, most sources acknowledge the existence of this model. According to Ted Beesley’s research, the SB-601 is a Pilot with a single active EMG P-type pickup. But, based on information provided by Willi Fritscher, who has managed the Guild factory for many years, it seems more likely that the 601 was a single
‘P-pickup’ version that featured the DiMarzio pickup. It was introduced in 1983 and discontinued at some point prior to 1987. In all likelihood, there were no SB-601’s made after 1984. Serial numbers begin with ‘BE’.
SB-603: This is the most vague of all Pilot models. It appears in some Guild records, but not in any catalogs that I have
found. The Blue Book of Electric Guitars identifies the 603 as having three pickups, but offers no further explanation as to type or manufacture. At my request, Mr. Fritscher (manager of the Guild factory) dug into his archives and discovered
that the 603 was a single "P-pickup" version (like the 601), using the EMG active pickup. He said that it was not a
popular model, so only about 50 were ever made. It seems that this model was first produced in 1983. And, it was discontinued prior to 1987. SB-603 serial numbers, I expect, would be preceded by the letters ‘BE’
