The 1851 Iowa State Special Census, Polk County, Greenfield Twp lists: line #19 William Allen (he is son of Wm M., Sr) line #20 Isaac Allen line #21 Whitley Allen line #24 William M Allen line #25 Jessie (Jesse) Allen
A.William Maynard Allen was my Great-Grandfather, b.2/1/1811,Watertown, NY. B. Both Whitley & Isaac are listed in the 1850 census of Polk Cty., Fort Des Moines Twp., Pg. 44. C.Wm M Allen is listed in the l850 Indiana census. Family Search CD Rom #17, Joseph P Allen (son) states moved to Iowa in l852.
It's strange that Allens moving to a new state would move next door to each other, unless there is a tie in of some kind.