Unless NEC planned to sell millions of each MobilePro model a year, there is no way it would design, patent & make a propreitary audio socket and plug. The cost of setting up a production line for just a proprietary socket alone is huge. I notice that "compatible sets" in "short supply" are advertised and sold on eBay and elsewhere for grossly inflated prices and a dishonest S\H charge.
The NEC 2.55mm socket is simply a standard universal cell phone socket that will work a mic\earphone combination unit. Many dollar stores, Dollar Tree for sure, sells a perfectly good universal cell phone mic\earphone set for $1.00. It is made by the same maker whose sets
(the same one
) is sold in different packaging at Meijer and many K-mart type stores for $9 to $12!
I have a dollar store universal cell phone boom mic\earphone unit that cost me a huge $1.00 that works just as well in my NEC 780 as does the unit a friend bought on eBay for $6.95 "buy it now" plus $5.95 S\H and his is an earphone unit only!
If you track the three clowns who "offer" their odd looking "earsets" for NEC on eBay, they never list having more than 5 or 6 sets at a time. I could have purchased 80 superior sets at Dollar Tree Wednesday for $1 each. They also call them hard to find "NEC MobilePro" compatible ear sets! I would never buy anything from any of these clowns. They are dishonest. I asked two of them, whose ads are very slick & vague, if their units included a mic. One told me he has a mic\earphone unit he could sell me for $29. I told him I would sell him as many of these rare units as he wants for $5 each. He was not interested. Imagine that!
...Of course, this is all just my humble opinion...
--Bruised
They had button batteries and 2.5 mm headphone jacks. I was a curious kid back then, and couldn't find a single set of headphones in the house that would fit, except the one that came with the radio of course. 
