Can you advise me as to what the problem is with my...

I have had a duplexing 1800 for some years during which all went well until about 6 months ago, when I could no longer duplex, having .received the message "duplex paper jam." Being a nosey engineer, I looked into the back and found a small, delicate plastic "finger" lying thereabout. Upon further nosing, including removal of a metal cover, I found the plastic device, off of which I was able to establish that the finger was broken. More recently, in attempting to establish a glue-on replacement to the broken finger, I demolished the item of which it was originally a part. Now, as an engineer for 55 years, I have come to look for the cause from which the effect derives; i.e., how come the plastic
finger broke off of the plastic part, and what was the role of the plastic part in allowing duplex printing or blocking it. After many blinding reflections, I came to the conclusion that the broken-off finger extended into the paper path for duplex printing and, from proper paper movement, signalled the printer control to allow backside printing of that sheet of paper. Absent the broken-off finger and the enabling signal, the printer stopped and produced the "duplex paper jam" message. I have not sought to locate and define other such "plastic finger" paper-sensing devices through which the printer's control monitors paper routing for one-side printing and for duplex printing. Suffice it to say that, in my bailing wire approach to fixing things, I apparently have further limited my printer to no printing at all and am facing its replacement. Anyone want a crippled NEC 1800? It is yours for the shipping and handling costs, and I will throw in my remaining inventory of powder-ink cartridges.