Short answer in relation to Elk serial numbers / PCB issue - we don’t know...
Members do list the serial numbers in this thread
, but not many say which board version they have.
Maybe you can spot a pattern in the reported serial numbers.
With BBC B machines, with the longer serial numbers, part of the serial number reflects the model and configuration as sold (disk interface, econet etc.). Part is related to the manufacturing plant or manufacturer that built the machine. The remaining section is presumably a simple incremental number. But it may or may not start off at 1 for each manufacturing plant or manufacturer. If it does not start at 1, we don’t know what number it started with.
With Elks I don’t know if there were multiple manufacturing plants or manufacturers in the U.K. Some were made in the U.K. and some were made in Malaysia. Obviously there was only one U.K. model.
It is possible to tell the issue 6 from the earlier issue boards without opening the case, but only if you have a RGB lead and can compare the video output between them. The issue 6 had more intense colours via the RGB output because a 74xxx series chip buffers the RGB video signals.