To compress both mailboxes into one, try the following: first load NTRAIN into the ACCUMULATOR; next, shift the ACCUMULATOR left 4 positions using the ASL command; then, use the ORA command with STRAIN to set the low 4-bits of the ACCUMULATOR; finally, store the ACCUMULATOR to location $4000.
Burn this program in an EEPROM. Also burn the original ECHO program in another EEPROM. You will use both programs to test the test-stand.