A few things...

Date codes from this era (first appearing, tbomk, third quarter 1934) use first number for quarter and second number for year... all in 1930's. 17 is first quarter 1937, just two quarters away from the barrel code.

In practice, nib codes often do seem to vary by a quarter or two from barrel codes in many Parker pens. Whether this represents parts sitting in storage vats for and being pulled up at random, or whether this subtle variation represents swaps at some point or another, who can say? I will speak for most of serious vintage pendom (eep!) by asserting that folks just don't mind this minor spread. In fact, many don't mind wider spreads so long as nib is correct size/model for pen. There are some shades of gray there.

You are not dealing with a pre-challenger nib. Challenger nibs, which appear to have much perhaps everything in common with Parker nibs used on some other sub-first-tier pens, were produced for the challenger since 1934 at least, and perhaps earlier for couple other models. Your 1937 nib most likely is correct model/style.

regards

david