You say that there is no model number on this revolver.... Respectfully, are you looking in the right place?
Here is where you will find the model marking...
Note in this case you will read "MOD.14-1".
Your Highway Patrolman looks to be a typical 28-2 from the 1970-1977 period. If you put up the serial number which is found on the butt of the revolver, we can narrow that date down a little....