It appears that the probe is still attached (it wraps around the back) but is missing its spring loaded tip... some sort of BNC connector has been cobbled onto the end of the standard probe. The standard probe is attached internally, and cannot be unplugged without separating the upper and lower parts of the case.
Are you missing the Vertical attenuator Knob as well?
Stan Griffiths in Aloha OR (W7NI) author of "Oscilloscopes-Selecting and Restoring a Classic" has been a good source of parts in the past. 503 649-0837...old number, might still work. Or the website below:
http://www.reprise.com/ash/clients2/home.asp
jr