Yes, that Code translates as 21 July 2003. The engine is a 28 cubic inch side valve type, made before production of most of the large side valve models was transferred to China. However the plant number of ZO mystifies me. It may have been some kind of interim number, since there was some juggling going on as most of the US plants closed one after another, in that period. Only OHV and DOV engines are made in the US now.
Here is the Operator's Manual for your engine:
http://bsintek.basco.com/BriggsDocumentDisplay/mssAIQ-K_ny7tH217FzoPu.pdfHere is the Illustrated Parts List:
http://bsintek.basco.com/BriggsDocumentDisplay/Z6jtAAVJ1DajI.pdfWith regard to your other points, Murray, Home Power, Simplicity, Snapper, Ferris, and Yard Products are all owned by Briggs. Hence your mower, as well as its engine, is a Briggs product. The lightweight center-engine machines like yours are not intended for heavy or commercial work of course, and are not really intended to have a long life, but there seem to be plenty of Rover and other products of that type that are 30 years old now, still in service here.