Jim,
The term "solid" was used to differentiate the true "brass" in the centerfires from the "pseudo-brass" or Brasslite coating on the rimfires.
Sorry about the confusion.
The brass color you see on a Henry centerfire extends all the way through the receiver.
The brass color you see on a Henry rimfire does not.
The centerfires are "solid brass".
The rimfires are not, and not even brass at all.
As far as the brass alloy goes, yes- brass in itself is an alloy.
The formulation that Henry uses (proprietary) contains more of other elements added to increase strength.
I won't say what they are, but I was surprised that there's still enough "brass" (as such) to retain the gold color & age like brass does.
It is a solid construct, no layering, no plating.
Best I can say, hope it helps.
It's just a much more sophisticated mix than merely copper & zinc.
Denis