"A" model harps were made in the US by Oscar Schmidt until about 1966, Give or take a year. Then came the "B" models, still 12 and 15 bar harps but with rounded corners and white plasitic bar buttons rather than the golf "T" style most of the "A" mosels used.
The 21 is a rather new style in the cosmic scheme of things. Felt and tuning pins are easy to locate. Call them zither pins and you may go further in your search. Many hammered dulcimer and harpsichord makers use the same tuning pins.