+-------------------------------------------+
|    Symbol characters 1-3 (Radix-50)       |
+-------------------------------------------+
|    Symbol characters 4-6 (Radix-50)       |
+---+---------------------------------------+
|   | Block number relative to start of file|
+---+--------------+------------------------+
|    Reserved      | Relative byte in block |
|    (7 bits)      |        (9 bits)        |
+------------------+------------------------+