I am watching the one with the tidal wave and there is little Erin looking so cute and small. So sad she and Buffy had such hard lives.

I was just watching / listening to Johnny Whitaker talking recently about the casting of Buffy Davis. John tells us that Erin Moran was being considered for the role, in addition to Pamelyn Ferdin (who I knew about from a previous interview). Of course as you indicated, Erin went on to make some appearances on the FA series, as did Pamelyn. Let's compare their years of birth with Anissa's:

1958 (Mar.) - Anissa Jones
1959 (Feb.) - Pamelyn Ferdin
1960 (Oct.) - Erin Moran

So we see that Anissa was significantly older than Erin. Well before her Happy Days work, Erin had a recurring role on Daktari in the 1968-69 season, 15 episodes. I consider myself a fan, she becomes a lovely young woman during the run of Happy Days.

Here's the late Erin Moran in a couple of promotional photos from the time of her regular role on Daktari, as a very young girl:

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It's odd that they didn't do publicity with Erin in dresses for Daktari given the sensibilities of a national audience of the time - particularly in the Mid-West and South.