The binary decus-5-8a seems to be EXP, though 
I could be mistaken.  Despite the checksum 
error, there's nothing obviously wrong with it.
I've created an annotated disassembly in 
exp.{pal,lst,bin}.

I haven't done anything with the tape image 
in decus-5-8.bin, as the checksums there are 
fine.

    Vince