A quick look, and I think it's an stp (super TP) tape -- stp is from the Harvard distribution. This would make sense, because that was the standard back before tar.
As Ron pointed out, tp (which Ken designed for DECTapes originally) puts the index at the head of the tape (tar and later cpis threaded the index inline). But it means its a fixed size and there were some other issues (tp may have originally been in assembler IIRC). On DECtape, tp worked pretty well/was pretty cool because you could update a block, less so on 9-track which when you re-wrote block N, you lost all blocks afterwards. Also, I don't remember why now [probably the limits off the directory], but it was typically in those days to take all the files in a directory, turn them into a foo.a (ar format) archive. So the stp image was a bunch of files: dir1/mumble.a dir2/grumble.a dir3/bumble.a ...
You then needed to unarchive the files within each directory. Also, remember ar(1) when through some changes in format between 4-7th editions as the compiler and linker matured. So watch out on that front too...
Anyway, v6 tp probably will read it, but if you poke around the TUHS and bitkeeper archives for the original Harvard distribution, stp.c should exist.