Dana,
My 'rule of thumb' for getting a letter is:
Gun is probably worth more than $1500, is older than 50 years and the ship date cannot be certain of the year, by the guys on this site. The serious collectors keep data bases on a lot of the more popular models and can bracket the ship date pretty close. I think most of your Ladysmith collection should qualify for letters.
Did you look under the grips of the target model for a S&W repair date? Should be a month.2 digit year with a symbol, indicating the date it was returned for refit/repair. If the target sights were added by the factory, it should have a date on the frame.
Regards,
George