The silver medallions date S&W's from around late 1929 and 1930 forward. The HD was introduced in April of 1930 and was S&W's first attempt to build a more powerful .38 Special revolver and hence the upgrade to the larger N frame platform. Basically it is a hot loaded .38 Special and was sometimes referred to as the .38 Special High Velocity or the .38 Super Police. The round is not to be confused with the earlier 38/44 round fired in the model 3's.
Eventually in 1935 the .357 Magnum was introduced on a custom order basis only and was referred to as the Registered Magnum.
Thank you for the info on the piece I knew it was something different.
Kevin