Well first off, as many people have pointed out, the rifle (if built in 1955) can not be original, as the 300 WM cartridge wasn't introduced until 1963. And caliber has everything to do with value for pre-64 model 70's...the most highly valued calibers (according to the 3 different 2012 value guides I have the top grade value is...) are (in order from highest) 300 Sav ($9400), 35 Rem.,458 African, 7mm, 250-3000 Sav., 375 H&H, .257 Bob., 300 H&H ($2500)....and so on from there. Pre-war rifles generally garner more than post war rifles such as yours.
If the rifle is a re-barreled action, which is has to be...the value is probably between $1000 and $1500 if it is in excellent shape and depending on which variety the rifle is...there are just to many varieties to list...but according to my books at the very least the action alone is worth $500 if it is functional and in working order.
You can write to the Cody (Buffalo Bill) Winchester Museum...PO Box 1000, Cody, WY and for a fee of $25 they will provide you a manufactures letter that will provide you the original data for the rifle based on the serial #.