Welcome...
Post some pictures when you get a chance, that will help jog people's memory.
Hallett Boats was started by Rich Hallett in the Los Angeles area during the early 50's. To the best of my recollection, he had allergic reactions to working with fiberglass and resins. He sold the company & the name to boat builder Nick Barron - Barron Boats Works, in the early 60's! Nick Barron still owns Hallett Boats today!
As for how many were made, you might try calling Hallett (http://www.hallettboats.com) and asking them.
Heres an Ad from 1963
http://www.v-drivevideo.com/jerrysga..._ads_078~0.jpg