Assembly numbers repeat and have nothing to do with date of manufacture or model.
The .38 HE and .32-20 HE, while being otherwise identical, are numbered in separate SN series. There is no correlation between models except, obviously, that lower numbers (in each series) were manufactured before higher numbers.
Until the early 1980s, when the ABCxxxx SN format came into use, each specific model had its' own separate SN series. As a result the serial number 123456 would have eventually appeared on several guns made at different times and of different models. Add the prefix letters they would/could have repeated again with different prefixes. Example, there could be (are!) K-Frame guns of different date and model with SNs of 123456, V123456, S123456, K123456, C123456, D123456, etc.
Make more sense now?