3 Speed Transmission Identification

[CA Victoria]

Information in VanPelt's V-8 Transmission publication shows two different 3 Speed top shifter cases.

I was under the impression that the transmission case with the long rib on the bottom of the case was a later production case. I pulled the transmission from my 50 pickup and noted that the transmission with the long rib had an 18-2,5xx,xxx serial number which puts it somewhere around 1936.

Based on this finding were both cases used interchangeably throughout production of the 3 speed top loader?