Hello,
The original design is, in fact, a large scale figure by sculptor Ron Tunison, specialized in ACW and Indian Wars subjects (he sculpts bronze monuments too). Andrea and Sieland copied it almost literally, down to the saddle on the ground (Sieland substituted a kepi for the original's hat, Andrea is more true to the source). The Tunison figure appeared, among others, in an old article (Dec. '91) about Tunison's work in Military Illustrated magazine. I would guess Andrea's and Sieland's "inspiration" came from there, unless they saw the original at some US model show.
I don't know which one came before, Andrea's or Sieland's; and I don't know if the second manufacturer in releasing the US Cavalryman was "inspired" by the first, of if both copied Tunison. What it's sure is that both came after Tunison work. Also, that Tunison's figure was better quality that both its imitations. Apparently they could not match his skills.
Not exactly creative work, then, by Andrea and Sieland.
Dani