it is one of two possible places. could have fallen out and been plugged so it wouldnt be an open hole(most logical) both would be on the passenger side of the engine. one is somewhere below exhaust port 3 or 4 in the block close to the pan(less old engines) verses on the timing cover where its on the passenger side down low on the part that kindof legs out also down close to the pan. (more 60s early 70s style) its just hard to say. might actually not have a hole, iv heard of timing covers that come that have to be drilled out by the buyer. (never have seen this so i dont beleive it) but who knows.
Do you have an aftermarket timing cover? I've heard the same thing that some of the covers need the hole for the dipstick tube drilled out. If it is original though then it should be low on the timing cover on the passenger side.