Thanks, I see the listing. Here is my notes on what tape head I was looking for: (it is about 3/4th of the way down the page; there is one photo and then one "diagram" I put together) https://voidstar.blog/ibm-5100-internal-tape-and-5106/ And I appreciate you taking the time to bring this to my attention (and the community in general). Tentatively, I'd say the ones you have aren't quite a match for the 5100-series. To explain: In the image/notes linked above, one photo has the tape unit head out with a blue label on it. That label says NORTRONICS P.N. 1608752 C584980 (then there are other numbers on it, like 269 and 24256). I mention these in text just since a search might happen across them someday, as opposed to images. Now across all this time, sure, we don't need EXACT part number matching. Across years or seasons, it's a good chance NORTRONICS changed part numbers. Recall, the tape unit itself was put together by 3M. The part number you referenced - it does seem to be from a "2-track" tape of some sort, so that's promising. But also in my link above, there is another photo - hard to describe, but it shows the "front" of the head. The discussion group I don't think would let me attach a small image? I've never actually tried. But anyhow, in that image: the head has "two halves". One half looks like an "upper and lower" for the two tracks, then next to that is a "vertical bar." I've never quite understood what that vertical bar is - I assume a WRITE portion of some sort?? The ones in the ebay listing don't have this "vertical bar." Then the pins on the back of the head: the one I have for my 5100/5110 is "6 small pins" next to "4 fat pins" (the 4 fat pins are in line with the "vertical bar" mentioned above). Of all the heads I came across, I could find the "6 small pin" configuration, but never the "4 fat pins." Now... in the past year or two, my trails have led me to getting two working tape units, and two broken ones. I've mulled over whether I should keep using the two working ones every now then (like once a year) just to keep tabs on them - or "lock them down" and never use them until I come across some confirmed tape (that has 5100/5110 content) that needs reading. But recently, I've been focused on a POLY88 restoration, but also the "new 8-bit system" the 6502-based Commander X16 (which its second round of pre-orders have done well, nearly sold out now). I've also learned about dishonest contractors recently, the hard way - construction related, not computer related; but it's eaten up a lot of time dealing with that. But my point there was: yep, I'm still interested in finding a working head. One other related note here: the earliest reference to the term "QIC" that I ever came across - after pouring through BYTE and PC Magazine - was around 1983 (early-mid 1980s), but NOT earlier than 1980. And I know some people don't like me calling those QIC tapes. But the description you provide in the Ebay listing is "track 2, single gap." Maybe. I noticed the 6 pins on the back of those being offered resemble the "fat pins" on the back of one mine (except mine only has 4 such pins, not 6). So I'll think it over some more if more information comes available, but most likely I may need to pass on the ones listed here. -SteveL (v*) On Tue, Oct 24, 2023 at 12:27 AM Mark Perullo via cctalk < cctalk(a)classiccmp.org&gt; wrote: