Looks like pull down door handles ? If so, it's a 51. I can also tell it's a 2 ton, by it having the heavier front axle, with bolt on front dust caps. All 47-53 Chevrolet 2 tons came with the Loadmaster 235 engine. All that said, it looks like you have a 1951 Chevrolet 6100 series, 2 ton, with short wheelbase, medium duty, Holmes Traffic King model 515 Wrecker. Holmes also had a model 460 Speed King, that used that same short Chariot Style service body, but different rear fenders. Should be a Ernest Holmes serial number plate on the passenger side boom mast vertical beam, to verify the model. That's a nice old wrecker right there. I've been collecting Vintage Holmes Wreckers, for many years. Here's the Heavy Duty Holmes model W35, I'm now working on.
Spanky Hardy Collector Of Fine Old G.M. COE Trucks & Antique Holmes Wreckers