This is because Toho classified Marvel Godzilla to be a variant of Showa Godzilla.
Topic on User talk:Les
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Where did Toho do this?
I got the information from this video.
www.youtube.com/watch?v=3U2WboIknJg&t=1s
Do not mean to intrude, but there is also this:
Right, that's what Slayer was referring to. But I think they're trying to relate it to the Godzilla from the '70s Marvel series.
Yeah, that seems to be the case. Thank you for the clarification!