I believe the reference was to Sanyo Cadnica N75P 1/3-N size cells. HP 41c packs I think had 1/2-N cells, didn't they?, but since those had all disappeared, the 1/3-N's were the next best thing.
But even the 1/3-N is dying. Sanyo is/was the last manufacturer supplying "N"-dimensioned NiCd product in the U.S., and a survey of their website seems to indicate that they have dropped all "N" diameters in favor of AA and AAA.
Some spot supplies of the N75P cell may still be around. I bought 16 recently at a Batteries Plus, and had them spot weld tabs on each (I already have a project for them, sorry). Since there are some 170 Batteries Plus stores in the U.S., it might well be worth a sweep in YOUR area.
And as Katie Wasserman has pointed out in her excellent summary (Articles Forum), 1/3-AAA NiCads are smaller and so will fit great. Their capacity, while not as large as the N75P, is probably very much in line with the old 1/2-N NiCd cells that were manufactured 25 or more years ago...
By the way-- other configurations of batteries may prove entertaining, if you're a designer-type. Varta makes button-cell NiCds; paralleling a couple of these to increase capacity and so forth, you might come up with an optimal packing of that plastic shell... :-)