some help in the datejust

[khengg]

i everyone. i am pretty new to the site. recently i made my 1st purchase for the SS SD & now i just can't get enough of rolex... already seeing for more.

i am planning to get a SS DJ for my wife.
seen a nice vintage 1550 SS DJ in good condition.

i would like to find out the difference between the original 1575 caliber as compared to the current DJ caliber.

also what is a market value for a vintage 1550 DJ.

thanks for your help in advance.