I'm not familiar with the Buckmark, so the similarities/differences between it and the Ruger are unknown to me and I can't make a valid comparison. The Ruger is designed such that the receiver detaches from the grip (frame) portion...it doesn't have a slide but uses a bolt. The serial number is on the receiver. In a very broad sense, it's much like an upper/lower combination with the serial # stamped on the upper.
Title 27, Code of Federal Regulations, section 478.11 defines a “firearm receiver” as, “[t]hat part of a firearm which provides housing for the hammer, bolt or breechblock, and firing mechanism, and which is usually threaded at its forward portion to receive the barrel.”