Somewhere in the 1980’s I have worked as a customer engineer at HP in the Netherlands. I still have some parts from these days. For instance your original 3/8Amp fuse with part number 1535-2586/7.
HP used light bulbs for detecting the end of tape in the technical HP79xx series. Maybe the HP85 uses a light bulb too. If so, I can send you some original bulbs (part number 2140-0221).
Let me know if you want me to send these parts.
I still have stacks of HP-85s, 86s, 87s, 87Ms, 9820s, 9825s, 9835s and 9845s, ONE HP 9831 and even a pair HP 9877 Mass Memory (Quad tape) drives. I even have one prototype HP-85 :-)
All of HPs (9825s, HP-85s, etc) that used that tape cassette used a physical hole on the tape to signal the EOT. But the problem is that the magnetic media flakes off of the (clear) tape and when that happens the calculator sees the clear spot and errors out with an "Unexpected End of Tape" message and the media is really, really, BAD about flaking off of those tapes! A few years ago I got an entire case of 100 (IIRC) brand new SEALED HP tapes and EVERY one of them was bad. I did find a few later tapes made by 3M that were the same size and I got a few of them to work but the HP tapes ALL went bad literally within a few days. I repaired a lot of those tape drives and I did have some success with getting them to work but I eventually gave up on getting them to work reliably due to the unreliability of the old tapes. If you really want to play with one of these old machines, get an HP-87 with the Mass Memory support, and HP-IB interface and connect it to one of the HP-IB 3 1/2" floppy drives. Those WORK! There ARE even some HP-IB hard drives that will work with them but it gets tricky finding a drive that uses the same command protocall that the old HP calculators use.
But FWIW my favorite of all the old HPs are the HP-120 and HP-125 64k CPM machines! Those little suckers work great with the 3 1/2" HP-IB floppy drives and are FAST!