What Nathan do I have?

Hi all. Bought this some years ago and put aside, but really don’t know what I have. Each horn is cast as follows:
P1=21056 p2=21057 p3=21058 p4=21059 base=21098
There is a cover plate over the fifth place on the manifold. If I find a p5 21060 would that make it a k5? Where could I find the p5? Where can I find catalog/more info? Thanks in advance for any help!