Okay, so your block size is set to 512 bytes, and somehow dd set to 512 couldn't read anything meaningful earlier.
778800 blocks 512 bytes each is 39GB of data - I guess that's compressed (DDS4).
try this then:
mt -f /dev/rmt/tps1d3nr rewind (will take a while)
dd if=/dev/rmt/tps1d3nr of=/tmp/file2 bs=512 count=778800
(you may need to try with count decreased by 1 and 2 as well, I can't remember if it counts filemarks as blocks or not).
This might be able to copy the whole tape to a file on your drive, if not, then I don't really have any more ideas.
Your drive seems to respond to commands, works with the tape, the tape DOES have the data, so it really should work with no problems...