The dies for 1921 were run hard and put up wet. Only the 1886, 1887 and 1889 Philadelphia mintages even approached
any 1921 mint production, and only the 1889-P actually exceeded it, producing more than Denver or San Francisco's 1921 outputs. The Philadelphia mint produced over 44 million 1921 Morgans, more than twice the production of any single year/mint. They were running the presses
hard.As a result, die cracks abound with 1921's. Like usual, San Francisco showed more attention to quality that the others, so relatively few S-mint errors reached circulation, and many of those are die polishing errors, showing their efforts to correct things. Denver and Philadelphia, though, ran the dies in some cases until they literally came apart, causing retained-cud errors. I think they'd lost the touch for producing such a large and complex coin in the years since 1904.
So, die cracks, even prominent ones, on 1921's are not really extraordinary. Of course, such can be said of almost any Morgan mintage - I've owned some pretty spectacular cracked examples which didn't warrant
VAM designation. With the 1921's, though, a pristine coin is almost the exception rather than the rule.
That's what draws me to them.

Even the one you submitted for me, clean as it looks, is covered with polishing lines under the loupe.