If you have a plus near one lead, it is an electrolytic capacitor. Aerovox was still making electrolytics in 1970 (see the date code). The replacement these days would be a plastic film capacitor. The 600 v will be no problem but you may have to buy a 0.47 uf or 0.68 uf one. Uf is the modern abbreviation for mf ( microfarad) as listed in 1970. Electrolytics were typically +80% -20% so unless this was in a tuned circuit ( and +-20 %) I would go with a 0.68 uf.
I buy capacitors mostly from newark.com (NC) or mouser.com (TX). On the newark website, click passive components, click capacitors, click film, click minimum capacitance .47 and maximum .68. Click minimum voltage 600, maximum 1000. Click stock only and click no direct ship (from the UK for $20 a line item). You will have a selection not always cheapest first. Mouser doesn't have minimum or maximum values, just exact values, but you can click more than one with the control key.
If this capacitor is in a motor run circuit, get a motor run or lamp ballast capacitor in a metal case. Typically made by genelec (newark) or epcos (mouser). Also a film dielectric capacitor. These small sizes there is no reason to put up with the wet process of electrolytics anymore. I've used 600 VAC rated film capacitors in DC service in my organ, at 450 VDC. The dc rating was not on the selector table, but the 47 uf 600 VAC genelecs I bought came in with a 1000 VDC rating printed on them. Rather large, make sure you have room for what you buy.
Paper in Oil or Paper in Wax, aluminum foil. Nice cap! 99% likely still fully functional and at or close to spec. Given the manufactured date and the carbon comp resistors it would take pretty big bucks to buy a newly manufactured PIO cap to replace it with comparable quality and sonic signature.
The old standard coding was C =Capacitor P=Paper. Some manufacturers used a O =Oil, some did not. Pretty much implied that if it was P, it was oil or wax. So the CP53B1 designates what it is, the dielectric and the form factor. The 504 designated the capacitance.
Thanks, that's the kind of info I was looking for. I suppose I didn't phrase my original question correctly. They are definitely not electrolytics, and they appear to be hermetically sealed. These are in a mid-70s Western Electric line insulation test set I came across for free.
So they could be PIO eh? I'm going to try them as coupling caps in a SE tube amp build.