That looks to be a 4 screw Pre-28 (Highway Patrolman) from around 1957

A bit more special than the common 28-2

It does however have the incorrect grips for the year.
It should have the "diamond" magnas on it, which you can find fairly easy on this forum from time to time.

As far as the price you paid - what I can is that if it had the original numbered diamond magnas on the gun - it would be worth more than you paid for it.

So I guess you did all right