"With the T added in 1958"
I don't think that's correct.
I have my Father's 521-T, which he bought new in 1949 or 1950, when he was 14 or 15. Somewhere around here I have the bill of sale.
The barrel is distinctly marked 521-T, and the barrel code is PTT.
According to this website:
http://www.remingtonsociety.com/questions/BLACKPOWDERX.htm, Dad's 521-T was manufactured in June of 1948.
Dad's gun also has no serial number.
To the best of my knowledge, Remington never offered palm rests for the 521. The 7-position rail on the forearm is for mounting the sling swivel and was to allow it to be adjusted for the arm span of the shooter.
As sold, the 521-T came with a 1" leather military style sling and a Lyman 57RS apeture sight.
I learned to shoot on Dad's 521. It was, and still is, extremely accurate.