My Lyman #43 lists both cartridges dimensional information as being the same. In fact, they are the only two pistol loads in the book that are listed under one single heading.
Lyman #43
Lists case head diameter (for both) as being .374"
Cartridge length as .675"

Speer #9 only lists the 380 ACP
The Speer manual lists the same rim diameter (.374")
However.
It lists the cartridge length as being .680".
Lyman doesn't state a rim thickness spec. Speer lists it @ .045".
Do you have any current vintage domestic stuff you could mic to compare dimensions off of? There's something about this issue rattling around in the back of my lame brain. Sorry, I just can't shake it out to the front?