A 420 20ga was my first double. Those Savage OU's were only made for a few years, just prior to WWII.
According to the information I have from an old Shooter's Bible, unless you've got a skeet gun--not likely--then your 430 will be choked M/F. And since the weight of the 16 is listed as being several ounces less than the 12ga, I'd guess that they're made on gauge-specific receivers.
The only differences between the base-model 420 and the "fancier" 430 was that the latter had a matted line on top of the barrel (neither had a rib), a checkered stock (the 420 had no checkering), and a Jostam recoil pad (420 had a buttplate).