Should be a machined groove on the balancer/pulley. My 64 Slant has only a single pulley. Remove the #1 spark plug (front) and jog the engine w/ a remote starter (or helper) while you hold your finger in the hole. Within 2 revs, you will feel it sucking a bit (intake stroke), jog slower since you will next feel the compression increasing. Start looking for the timing mark. It should be just after the compression stops increasing. Make a chalk mark so it shows up better with your timing light.
It is possible that the rubber damper slipped and it isn't in the correct place. You can still set the engine fine, by adjusting timing for optimal vacuum, which is what everyone does anyway, you just won't know what advance you have.
You can find TDC even closer by sticking a stiff wire in the hole to sense the piston (bend so it won't fall in!). Even better is using a piston stop. For these, turn the engine over by hand, by tugging on the fan belt or use the crank bolt.
