Gross Weights
 

I have recently become a bit confused regarding the different Gross Weight certifications on Skymasters. In reading the history, the GTOW has varied significantly from less than 4200 to 4630. My question for anyone who may know the answer is this: Given the same type (i.e. non-pressurized, 337), if the design hardly changed over the years, is there any reason why the GW changes so much? Said another way, is a 70s model really stronger than a 60s model?

Yes
The 70's models use different spars, hence the longer duration until spar mod. They use different tires, to support the greater weight.

My 69 has a GW takeoff of 4500, and landing weight of 4400

The later models are narrower inside, have different doors, and supports, no baggage door, so there is different plumbing.