Graticular Blocks (WA)
http://www.dmp.wa.gov.au/documents/132289_Graticular_Boundary_System.pdfA graticular block is one minute of latitude by 1 minute of longitude. This is called a sub-block outside of WA.
The National 1:1Million mapping series is broken into 5min x 5min blocks, which are numbered 1-3456, and then divided into 25 1min x 1min sub-blocks with the letters a to z, excluding i.
This Document describes graticular blocks in WA.