My x250 is out of commission for now, so I got a T540p. I have customer complaints that DOS does not work on any machine newer than T43. I don't use DOS myself, so I can't easily troubleshoot.
I don't have 123.EXE, I have tried the install disks. Disk1 still gives the error listed above in the DOS Full Screen.
I think the L420 would be new enough if it only worked. But not being a DOS user, I have no idea.
Others are trying UEFI, which is good. There ought to be a way to do this, but often so often, DOS users cannot or will not use Internet.