Never having had an SBT, or really spent much time looking at one, I don't understand why the ejector hammer is split. Could someone explain that for me?

"Why not keep costs down and use available parts from the doubles production rather than develop an entirely new ejector system for a low production gun?" was probably the thinking of Parker Bros.

Plausible explanation, though generous. By 'system' I'm assuming only the hammer would differ.

It is two hammers (identical parts as in the double), but they are pinned together and operate as one.