Hi MrCJ, what you have is a Model 5 series 3 (no edger groove slot) with an earlie Model 4 engine as Blue has already stated, $20 is a good score, I would say the cowel that is on it now would be the correct one as it has the rubber feet for the petrol tank to sit on, it would have had the rubber strap under the steel band that holds the tank down also, non original wheels, need the white wheels as in your pic with the red hub cap, I would say the starter was from a bit earlier model (probably Model 4 as your motor is) cannot see a lot more I can tell you with those pic's but you got a good score as I (and others) like the look of series 3. so

engine year as Blue said 58
Mower 59-60