Since it appears that you know where the sewer line exits the house, the septic tank in generally in a straight line. Unless there is a some sort of special reason, i.e. the lay of the land, the tank generally should be within about 10 feet of the house. If the probe bar is too large, try bending a piece of rebar to make a handle and sharpen the end to make probing easier. The pipe willl be at the same dpeth as the exit from the house and is about 4 inches wide. Once you find it, move along it until you find something wider that four inches. This should then be the septic tank. 

Since you say that there is no septic permit on file, is it possible that this might be a homemede system? If so, then perhaps there is no septic tank, but a cesspool. You should still be able to find the pipe connecting the house to the pit. 

If this does not work, consider looking in the yellow pages for a septic pumper who uses a radio transmitter to find pipes and seeptic tanks. This can be introduced from the house and followed electronically. It can determine both the direction and depth. Once the tank is found, the same method can be used to locate the drainfield.

Septic contractors in your area can help you better understand what methods of construction might have been used in your area in 1965. If you cannot find the system, it probably means that the septic tank has not been pumped since construction, and certainly need to be pumped and inspected. Good luck. Please do not hesitate to call if I can be of further assistance.