I have a 1894CP & I noticed on the left side of the receiver(Opposite side of the ejection port) there is what looks like a plug, is this a pre drilled hole for a receiver sight?

It's for the ejector stud. If you remove the bolt you'll see the wishbone shaped ejector sitting in a slot on the left side. Remove the ejector before you clean the barrel, or it will snag on a cleaning patch. If the receiver was drilled for a receiver sight, there's be three holes insteads of one.

Bye
Jack