Earl Caldwell is an amazing story.  Like you said, came up in 1928, came up again in 1935 (outduelling Schoolboy Rowe in his second MLB "debut"), came up again in 1946 and had a career year at the age of 41.  I think the most amazing part of the story is that he led the Gulf Coast League in ERA two years running at the ages of 46 and 47.  Art Fowler had a vaguely similar career trajectory and had his best seasons with the Angels when he was 40-ish before he hooked up with Billy Martin.  And the Dominican pitching legend Diomedes Olivo (Chi Chi's brother) was the second oldest modern day rookie (behind Satchel Paige) when he broke in with the Pirates at age 41 and had a couple good seasons at the end of an amazing career in Latin ball. 

And yet another reason to love progressive leagues.  I bet that 99% of the people on this site had never heard of Caldwell prior to this post.  The same 99% would probably overlook him in an open league, or, if they did take his good season, would never really look at his overall career outside of that one time use.

Trivia question: What present did Earl Caldwell get for winning a game in 1945 (which turned out to be his 5th, and final, career shutout)?

Ha.  A hen.

From The Bill James Historical Baseball Abstract:

Playing center field at Comiskey Field on August 3, 1945, [Doc] Cramer was astonished to see a chicken running loose in center field.  Cramer chased the chicken all over center for several minutes, finally caught it, and discovered a note attached saying, "This chicken is a present for the pitcher who wins the game."  Earl Caldwell pitched a shutout, and Cramer handed him the chicken as he left the mound.

I wonder if Caldwell wasn't a spitballer, who, like Frank Shellenback, did not get "grandfathered in" so he could continue to use the pitch?

to late, the spitballer thing was in 1917 or 1918....  Caldwell was just a guy who wanted to play baseball (or didn't want to sell tires)