The FBI report on the Togo timer does not mention the watermark on the main board of the timer. This is not surprising as the report signed by Thurman is almost a copy of the CIA report. (Here is this undated FBI report. I think that it must have been written between 26 September and 11 December 1990.)
However, a FBI document dated 20 August 1990 mentions the following:
“Special photography techniques were utilized on specimen K-1 to identify the number/letter “21B” which is contained within the figure of an oval. The number/letter appears in the upper left quadrant of the circuit board, within the lamination and not on the surface of the board. This identification was added during the manufacturing process, and may offer a code to identify the source of this board. It should be noted, however, that the company which applied the actual printed circuit, may not be the company which manufactured the green circuit board.”
One may perhaps honestly confuse “MEBO” for “M580” or “MEBQ”. But why would anyone misread “SIB” for “21B” when the “B” is asymmetrical is puzzling to say the least…
But let us not get distracted and let us keep on going with the main goal: Can we date the earliest possible time of PT/35(b) fabrication?