I took a look at the parts manual for the 250 C, and it shows the speedometer gear at the rear of the manual transmission.
Warning - assumption coming - would it be the same type of set up as the Pagoda? Where there is a clamping type connection at the rear of the transmission where the drive end fits into. There is a half-moon shaped piece that fits into the concave portion of the drive end of the speedo cable.
The photos are from the 113 manual, but it should give an idea of the connection. (Please excuse the gravity defying orientation)