yep there is a freeze plug behind the starter and its probably the culprit. they look fine on the outside but you can poke your finger through them if rusted away on the inside.
I bought a one owner 67 in 04. He had added one bottle of barsleak per year for 9 years. Chunks of that stuff had set up in the back end of the block.
He put on a weak system cap ( 7 lbs. ) and rarely had to add water but aways had a few drops appear in the garage after his short weekly drive to the golf course.
I drove it over 2800 miles comming home and never needed to add water.
I discovered this leak to be sooo small that the water evaporated when engine was running.