I was web searching on this same subject and found a manual for a MFJ 247 which interestingly enough says:
"If you want the use the counter to measure other than the 247's oscillator, attach a counter probe or a piece of wire to the BNC connector at the top right of the case.
NOTE: The sensitivity of the counter is about 600mV. A "TIMES 1" probe must be used unless you are measuring a very large signal."
FWIW. Probably has similar innards to the 249.http://www.mfjenterprises.com/support/MFJ-247/MFJ-247.pdf