Brother HL-1050 Technical Reference Manual page 501

The 5th byte data onward is replaced by the twelve data bytes AAh.
11101010b,02h,Aah
When the replacement position and the amount of replacement data are overflow, the next data byte
becomes an Expansion Offset Byte. However, if the number is FFh, 00h is added.
The number of command bytes = 300, FFh+2Dh
The number of command bytes = 510, FFh+FFh+00h
If there are more than two command bytes for 1 raster, the replacement position for the second command
byte onwards is indicated by the offset byte, starting from the data position replaced by the first command
byte.
APPENDIX C HBP MODE COMMANDS - 12