As for pinning down any change in design features, I have never seen an instance where there was a clean break in the serial number range. There is always some overlap. Many collectors attribute this to the "bottom of the bin" theory, which makes sense. As a component went through the process up to serial numbering later configuration parts likely were placed in the pile and some new configuration components were completed prior to the previous version. Also as production caught up with demand parts were shelved until needed to meet orders. On the centerfire sporters for example, receivers for less popular or more expensive variations laid in inventory for decades before being pulled, serial numbered and built into rifles.