Two measurable things: 1) the distance between the buttoned strap and the earflap, measurable in units of strap-width; and 2) relative width of facial trim, plus b/w contrast of trim vs. leather (even acknowledging photographic element contrast effects). In the 1928 cap the strap is farther from the earflap, and the facial trim is much wider and brighter in contrast to the leather.
There are also logical inconsistencies regarding need/evidence for AE's use of caps during 1928 vs. 1929 flights, but I'm resigned to stop stirring the soup on this topic.
Dan Brown, #2408