There was always two BH frame sizes; the OM mid size for the 357 mag and large frame for all other center fire calibers.
The NM BH 357 has been built on the large frame since the NMs were introduced in 1973. However, in 2005 the NM "Flat Top" BHs are built on the mid size frame for all calibers except the flat top 41 and 44 mags. The same is true for the New Vaqueros.