Both of the blue Crests are plunger fill.
Well, I did more reading-- recognizing some of the limitations found even in those very nice 1947 and 1948 Workbooks-- and I remain without perfect answer. Indeed my headache grows.
We know there were $17.50 Crest Deluxe and $15 Crest during 1945-7 celluloid era. The Tucky seems to be Crest Deluxe only at $17.50 during 1945-7.
Same years we had Sentinel Deluxe at $15 and Sentinel at $12.50 with the Tucky (unlike for the Crest cluster) being NON-Deluxe at $12.50
But the 1947 catalogue shows only the larger size (price) pen for the two lines in non-Tucky: $17.50 Crest Deluxe and $15. Sentinel Deluxe. I'd speculated the smaller clip-version pens had been discontinued before the 1947 catalogue was issued.
However, the injection-plastic era 1948 catalogue again shows larger and smaller clip pens in both Crest and Sentinel family. Crest Deluxe $17.50. Thinner Crest $15. Sentinel Deluxe $15, thinner Sentinel-- if the catalogue is to be believed-- also now $15.
Yet, with another "however", I note that the 1948 catalogue for Tuckaway shows still just one size (as with prior catalogue) for Crest and Sentinel, still calling the Crest a "Crest Deluxe" and still calling the Sentinel just 'Sentinel". But, for Sentinel Tuckaway, as with the full-length Sentinel, proper, the price listed is $15 instead of the earlier 1945-7 $12.50.
Offhand I do suspect the 1948 Crest Deluxe Tucky and Sentinel Tucky were the same size, despite one having the "deluxe" label, the other not. If so, then the likely explanation for a metal cap, white-dot-on-butt, 1948 pen with 1500 code but GF cap is that the Sentinel Tucky barrel was mated to a Crest Deluxe Tucky cap.
All this assumes the catalogue didn't get the prices wrong (if we find a Sentinel with 1250 code that would be food for thought) or that Sheaffer just didn't screw up an imprint on a pen here and there, leading to odd finds in the wild
But, Crest Tucky (formally "Crest Deluxe Tuckaway") was a $17.50 pen even in 1945-7, and I see no sign any Crest Tuckies from 1948 were described as $15 pens.
Again, this decade at Sheaffer is a headache.