In March 2000, researchers from Dundee University conducted a series of tests on PT/35(b) and DP/31. Here is their full report.
Conclusion of the Report
The fragments PT/35(b) and DP/31 are from the same physical printed circuit board.
Comment on the production of PT/35(b)
Both samples of printed circuit board (PCB) indicate that the copper sublayer has been coated with tin using a tin immersion process. This is evident by the uniform silver like deposition. This can be easily undertaken by immersing the board into a commercially available liquid chemical, which is the process used for small-scale manufacture of PCBs.
Note about the appearance of PT/35(b) and DP/31
The fragments look different. PT/35(b) appears greenish while DP/31 appears brownish. This is explained by the fact that DP/31 has been modified. (The green back lacquer has been removed as well as 3 layers.)