Photo shows what I believe to be a chronological progression of mold types from L to R. Far left is SDP. Bob Stahr submits that the RDP's began production in 1925. Based on the widespread production of Hemi Blue SDP pieces, one might assume that the piece 2nd from left is the earliest, followed by aqua, ice and then with the addition of date codes in 1934.

The two-tone clear/blue piece (3rd from left) presents an anomaly in the lineup for both color and mold profile.