Any idea on what system the tape could have been created? Do a DMPTAP command to dump the VOL and HDR labels to print and have a look at this. One of the HDR labels usually tells you the source operating system or source system. I think you might already have done this.
If the tape was created on an AS/400 try various options of the DSPTAP command.
On the HDR labels you should also be able to see the format the tape was written in – F, FB, V, VB, etc. This might help. Have a look at Information Center for a layout of the VOL and HDR labels.
Please let me know what happens.
Best of luck