My guess:
D197
D = April (Dth or 4th month)
19 = day of the month
7 = 1957
You'll never know if it was the original engine unless (1) you have the original title/registration, AND (2) the original title/registration has the original engine number as the VID/VIN (it could be an engine code or a door-post-plate code).
Now, do you want to check to see if the head date-casting code is also for 1957?
