note: this is a continuation of rl2doc! see that first for more context

opl3 data table entry
offset    | size     | token | description
----------|----------|-------|--------------------------------------------------
        0 |       28 |       | opl3 parameters


instrument entry
offset    | size     | token | description
----------|----------|-------|--------------------------------------------------
        0 |        3 |       | ascii signature 'O3_'
        3 |       12 |       | null padded, null terminated instrument name
       15 |        5 |       | unknown (always 01 00 00 00 00)
       21 |        2 |       | offset into file (inside opl3 data table)
       23 |        2 |       | unknown (all zero)
       25 |        2 |       | unknown (either size of this table or size of
          |          |       | opl3 data table entry; always 28)
       27 |        2 |       | unknown (either size of this table or size of
          |          |       | opl3 data table entry; always 28)
TODO: offsets are wrong! sizes are correct

main header
offset    | size     | token | description
----------|----------|-------|--------------------------------------------------
        0 |       30 |       | ascii signature 'Accomp. Bank, (C) AdLib Inc\x00
          |          |       | \x01\x01'
       31 |       10 |       | unknown (all zero)
       41 |        2 |       | multiply by 28, then add 48 to get offset
          |          |       | to the start of the opl3 data table
       42 |        2 |    is | number of defined instruments
       44 |        4 |       | unknown (all zero)
       48 |  [is]*28 |   int | instrument name table