Larry is on target, but I would add one refinement. In 1950 the company switched from the long N-frame action to the short action. I would say the true Pre-20s would be the short actions guns beginning in 1950, while the late '40s .38/44 Heavy Duty revolvers would be considered "Postwar Transitional" revolvers.
You can tell them apart by their hammer profiles -- the short action "Pre-20" revolvers have the low hammer spur still seen on S&W revolvers today. The transitional guns have the high spur hammers that look very prewar in their design.
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David Wilson
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