Kxtvet did answer your question, he said the DBS
There is like most Panasonic systems different sizes in that range.
They are an old system and just because he has a load of handsets does not make it a good idea to supply him with a ksu Are you going to give him warranty on that unit as is required by law once supplying a second hand unit

“I have not failed. I've just found 10,000 ways that won't work.”