Like frank said, they both are Repco, and I believe both have Mercury applications (along with others). The square Phelons seems to be quite common for the Mercs … I’ve used them Mark motors and some green tanks. I have a 1968 Mercury 20HP that takes the round Phelons, and I think a KE4 I just refurbished and a Scott-Atwater 7.5 (I was expecting Wilco) also takes the round Phelons. If I am not mistaken, the condensers matched to them differ, but I’m not sure if they actually have different resistance ratings, or they just have different mounts.
Sorry this isn’t a more detailed response, but hopefully there is something useful in it.
JP