If you don't find a tag you can look on the block where the oil filter adapter is and find the date code of the block. It won't tell you exactly what it is but the date will get you in the ballpark. If it's dated late 60's or early 70's you'll know it's not original. If you don't know how to read the code just post up what you've found and we'll decipher it for you.
Later 60's & 70's had the tag bolted to the coil bracket.

Here was some additional info on the car the current owner texted me. Was thinking of an offer around $12k. Interested in hearing everyone’s thoughts.
“It should start, had it running to load. The shifter is mounted with 3-5/16 machine bolts. 1-2 missing, 1 sheared off. Was going to pull trans and do clutch too
I think it's currently in 3rd gear
Clutch was starting to slip under hard acceleration, could use rear seal. Trans is sound gear wise. Very short throw mr gasket shifter. Have original shifter. Have extra block with crank that didn't need, also original intake/autolite carb
Engine is oily but only around 7500-10000 mi.
Full overhaul, original heads re worked, sealed power pistons 4 valve relief, not sure if they're flattops.  Performer cam/int/750 carb.
The black on underside is factory undercoating.

Looks like the engine date code is in the range for 64. 4(64) B (Feb.) 12 (12th day of the month) Th e cyl head engineering #'s the same but he misread it. Should be C4AE 6090 O. I would double check the last "O" to make sure. There's also a date code on the heads between  the middle valve springs. I would think they would be close to the date on the block.
Hard to say what it's worth without at least seeing a picture of it. Sounds like it needs some work and you could open up a can of worms.
I'd start low on an offer, you can always come up.

Quick Question, the Door Tag Vehicle Warranty number is 4Z68Z121415. The VIN on the title is 4G69Z160437. The owner said he'll send over a pic of the official Vehicle Identification Number for title and registration purposes stamped on a tab attached to the top right side (weld flange) of the dash panel in the engine compartment. Would those numbers be different? Just want to make sure I don't have a "put together" car that's not all original.
Thanks, Russell

The owner is saying the VIN listed on the title is ok even though it’s different than the one on the VIN tag & door tag? Something about Ford using three different numbers?? Doesn’t sound right to me. The VIN off the title appears to be for a 64 390 Convertible.