Nib grade
A flexible nib spreads its tines under pressure, so the line swells on the downstrokes and thins on the upstrokes. Plenty of pre-war pens had softer nibs than anything sold today, and semi-flexible gold nibs turn up on older Pelikans and Montblancs. Most nibs labelled flex on eBay are just soft, though, so ask for a writing sample that shows the line width changing. Never test one by pressing hard. A nib that's been pushed too far springs out of shape and won't close back up.
2 pens we cover.

Pelikan · 1956–1965
PistonThird version of the 400, 1956 to 1965
Post-war golden age
$260–$340 typical ask
24 on eBay

Pelikan · 1929–1944
PistonThe first Pelikan, celluloid barrel
Pre-war and wartime
$400–$550 typical ask
10 on eBay