insulator 434, plug metal casing has the last 5 of the plug number.

So we did find a vintage catalog. Here is the 6 digit part number chart from the book.

First: 2 Standard plug, 3 Resistor plug
2nd: Thread size 1=10mm, 2=12mm, 4=14mm, 8=18mm
3rd: 1=gasket, 2=taper
4th: Heat range. 9=Hottest, 0= coldest
5th: Reach: 1=1/4", 2=3/8", 456.....7=3/4" and 8=3/4" with 3/8th thread
6th Electrode: 1=Regular; 2=Extended, 3=Multi-Reg, 4=Multi-Extend, 5= Platnium, 6= Recessed

So this is a regular tipped version of the hemi or small block, in a pretty hot range. Champion 16 equivalent.
The necessary plug would be a 241X72. With X being heat range.