Info from 'The Rifleman's Rifle' by Rule ,
Date marking (hand stamped 2 digit) dropped in 1956, but occasional barrel will be found with later date stamp. Indicates year the bbl was made (not necessarily same as the year rifle was produced).
Caliber marking on the bottom of barrel dropped in 1957.
But again occasional examples of later production rifles show up that were assembled with earlier barrels.
Standard production sling swivels were non-detachable.
Most rifles delivered with 1" swivels. Some had 1 1/4" swivels, those being found mostly on heavy bbl rifles.
No changes during production.
Super Grade rifles had QD swivels/bases.