The real solution is to make sure the source and target versions of the LCT file actually match!
Source was 115.39
Target was 115.40
Oh, and according to Metalink, the correct use of the optional parameter on FNDLOAD is...
FNDLOAD apps/apps_pwd@env 0 Y UPLOAD afcpprog.lct my_dat.ldt - CUSTOM_MODE=FORCENote the subtle extra '-'

A combination of the two has got the LDT file to load.