there is no connection the old multiverse was Earth-One well the graphic novels are Earth One however the date of the publication date of 2012 (for Batman Earth One vol 1) puts them in the new 52 era. the designation earth One was given to it because they changed the primary earth designation from Earth-One to Earth Prime/0.
for example in the new earth one history this earth batman’s mother is not born Martha Kane But instead is born Martha Arkham and wonder woman is the daughter of Hercules and Hippolyta through some strange amazon science.
also, we need to remember (at least as i understand it currently) that it was revealed at the end of flashpoint that there more like 3 main fifty twos in the DC universe which would be the Golden Age 52, Silver Age 52 and New Earth 52, now there is the Earth-Prime 52 (new 52) which now makes up The Metaverse with the end of Death Metal…However, there is also likely a Flashpoint 52. and death metal returns these worlds into a single massive multiverse.
given the Source Wall literally did not exist until 1971 that would in theory mean the core DC 52’s multiverse was severed from a greater Metaverse at this time.