I have a 5100 and 5106 both with a working tape unit. But the tape unit in my 5110 is not working, and I believe it is the read/write head. The REWIND command works, but the MARK command does not (ERR 3). I took the Tape Card from the 5110 and swapped it into the 5106, and the 5106 continued to work (and 5110 then had same result, ERR 3). This tells me the issue isn't any electronics in the Tape Card electronics (such as bad IC's). Swapping the electronics card is easy. But I haven't yet tried taking the read/write head from the 5106 and putting it into 5110's tape unit (they both use all the same components) -- it's not that difficult, but I don't really want to put a working 5106 at more risk of becoming non-working. Hence, that's why I'm asking around first if anyone might happen to have one of these NORTRONICS or a suitable substitute to try. For example, it may be the same kind of read/write head used in some reel-to-reel systems of that era (early/mid 1970s). The various numbers on the read/write head are as follows: NORTRONICS PN 1608752 C584980 269 24256 I have measurements and photos of what I'm looking for at the bottom of this link: https://voidstar.blog/ibm-5100-internal-tape-and-5106/ The head is about a 15mm x 20mm, with a 26mm ground extension on one side for mounting. A few months ago I contacted Nortronics. They had some stock that was close, but not this exact pin out (4-pins on one side, 6-pins on the other; not sure which is read vs write). I think all the ones they had were 4-pin/4-pin. I've also tried different brand tapes, DC6150 vs DC600A (which all of them do work in the 5100, but not the 5110). -Steve