What's happening here is the tape commands use ioctl calls to query the tape device. Ioctl commands are just special commands that a device driver can use to get information from a device. For tapes, the minor number for ioctl is 128. Some tapes require formatting, some have the ability to be retensioned, some don't. The first problem you can run into is hardware compression.
Different manufacturers can use different schemes.
Therefore, if you are planning to transfer to another system, you want compression turned off. To do that on OSR5, you'd say tape -a 0 setcomp Another area of problem is hardware block size. The defaults can vary, so you need to explicitly set it to match. Consider making a contribution to help pay for the costs of bringing it to you. Or you may also use Paypal to make donations.