Is it the serial number or the assembly number that is stamped on the frame that is covered by the yoke?

Why the question about it’s value? It’s current value is what you paid for it. There is no way to accurately predict it’s future value.

It sounds like the O.P. is having a case of buyer’s remorse. Since the seller is offering you a full refund returning the gun back to him would relieve you of any worries about it.

p.s. you do realize the the oversize target grips were not standard for the Model 28.