@Neil
I use portable Ultraedit 32:
Spoiler: CLICK
I search 105 ( 31 00 30 00 35 and 31 30 35 ) but can't find it in FW, then i search for -0 ( 2D 30 ) and found in both FW of 3xx A0083-00.
When i look this in FW of nuvi 2x5 i also find this string.
After this i start again search for -0 ( 2D 30 ) and scroll up when i found it in FW.
That is how i found XBLKS and missing PCB number :)
If you search 2D 30 in nuvi 310 FW your first hit be -043 and second search you will see A0083-00, now scroll up and you will find XBLKS