Hi, Barry;
just to add a few info to the ones Andrés and Raymond have already posted. I was lucky enough to collect two sets of these contacts - two brass (golden platted) balls, two springs (also golden plated) and a rubber support - and I use them in two 41 calculators: a CV and a CX, both fullnut models. AFAIK, all fullnuts are ready to use these contacts, but a few of them have the set itself, as mentioned by Andrés.
I use an external pack with four D-size alkaline batteries to power the calculator when using the card reader (and/or the optical wand) without worrying about discharged batteries. The worst case is the starting of the card reader pulling engine, a peak of current that exhausts the N-cells quickly. To power the calculator with the external batteries I use a home-made contact, a single-sided PCB conveniently cut. The copper surface is periodically polished and I managed to separate it in two parts ([+] and [-]) with a single cut in the middle. I added a plastic fastener in the isolating surface so both balls are pressed through the copper surface by the springs.
If you plan to use your HP41 as a "stand alone" calculator, a fresh battery pack will rest long enough so you'll forget it needs replacement (as Andrés mentioned, too). In fact, when you are a "first timer" in an HP41, the battery pack may rest for a few months only, because you'll want to use your HP41 too often... Never mind, we all here know what does it mean.
Congrats! You found a piece of the best Engineering available for its time that still amuses many people.
Luiz (Brazil)
