Could be an uncommon variation of #13, but the color is similar to the #52 Five Gaiter, with grey hooves and bald face. Some models have eye whites (most eye whites had ended by 1969). That color of the Five Gaiter was around from 1963-1986. Unknown if any of these models have the USA Stamp; however it does not appear the mare had the USA Stamp.
Overlap production years between #13 FAS and #52 Five Gaiter is 1963-1973. Estimated these were made sometime from 1963-1969 (based on the Mare not having the USA Stamp).