These people can do it:
The output cable from the phonoDo you mean from the turntable to the input of the amplifier? That would be straight from the cartridge which you can't just feed into the computer. It needs to be amplified a bit and equalization applied. In other words you need a phono stage. You also need a catridge designed to play 78s.
The output cable from the phono which connects to the rest of the system is a round connector with three pins.One common signal ground pin and a right and left channel signal pins.
Is the cartridge a ceramic type or magnetic. If it is a ceramic you will not need a phono preamp as Herman described in his post. If the cartridge is magnetic you can pick up a cheap outboard phono preamp at Radio Shack.
Ceramic cartridge example:
Thank you all for your guidance (and the Flashback Sales link!). It looks like these connectors are still fairly common in Europe -- I even identified my mystery speaker connector at the Flashback site there.
The cartridge is a ceramic cartridge (Telefunken T200). It has a diamond needle -- one of the flipover kind that has the 78 stylus on the other side.