The diagnostic T020C tests writes at the end of memory by writing from x9990 to x9999 expecting 10 characters to be written. However the 1410 logic asserts internal end of transfer just as the address transitions from x9998 to x9999, causing a wrong length record (WLR) status, and also cutting the number of characters written down to 9. This causes subsequent reads to come up 1 character short as well, so they also cause an WLR indication, as there is no GMWM at location x9999.
This appears to be how the logic existed in 1962, but the diagnostic is from 1969.
The diagnostic T020C tests writes at the end of memory by writing from x9990 to x9999 expecting 10 characters to be written. However the 1410 logic asserts internal end of transfer just as the address transitions from x9998 to x9999, causing a wrong length record (WLR) status, and also cutting the number of characters written down to 9. This causes subsequent reads to come up 1 character short as well, so they also cause an WLR indication, as there is no GMWM at location x9999.
This appears to be how the logic existed in 1962, but the diagnostic is from 1969.