Late, series 40 rifles exhibit what we collectors call SLD (flat-top front band, recessed bolt stop, etc.) and 'conventional' last ditch features. These changes are not sequential with serial numbers. SLD features appear with serial numbers lower and higher than those of the 'conventional' last ditch rifles. Since tooling is different; were there multiple assembly lines, drawing in different tooling: or, were parts being used at random from a variety of small suppliers? My thought is that the second option is not viable logically: It seems that flat top front bands usually have all the SLD features; while round top front bands have 'conventional' bolt stop, etc. Serial number seems to have very little effect on the appearance of these features (other than they are all high-number Jinsens). Any thoughts on this topic?