I just got my 2016 catalog from Lyman yesterday.
Why are there molds for .381 dia?
Whelennut
These are supposedly for casting 38-55 bullets as many run that big in bore diameter.
And for the .38-90.
To the OP, if you haven't decided on a mould yet, check out Montana Bullet Company's offerings and then choose your mould.
LINK I have an old custom SAECO .375 GC, spitzer shape, double cavity that throws a 265 gr bullet of wheelweights. That seems to be an optimum bullet weight for .375's.
In case you didn't know, they aren't making SR-4759 anymore.
Ed