Looks like these are the images that leaked his reveal in the first place https://www.cbr.com/godzilla-vs-kong-toys-larger-kong-mechagodzilla/
"Who will be using Mechagodzilla and for what purpose are questions that don't have answers - yet, but the credits of Godzilla: King of the Monsters may offer an explanation on who builds Mechagodzilla. Carefully hidden in one of the newspaper articles in the credits, is a brief line about Monarch building a "mechanized giant" on Skull Island. This "mechanized giant" may turn out to be Mechagodzilla. Also, this giant's locations coincides with the setting of Godzilla vs. Kong, since at least part of the movie will take place on Kong's home."
I think there’s a chance the “bad” Godzilla is actually MechaG as well, given that’s how it went down in ‘74.