If you moved, allocate the correct portion of the total to each state.
In other words, if you have 25,000 in Box 1
Lived 6 months in Illinois, and 6 months in Michigan, enter 12,500 for Illinois Box 16 and 12,500 for Michigan Box 16.
You are correct that a CSA 1099R has no box 16, so it would be up to you to allocate the correct amount of the total to each state.
Divide the total by 12 and then multiply that by the months in each state.
Example: Total in Box 1 is 24,000
Divided by 12 is 2,000 per month
lived in Ohio 4 months equals 2,000 X 4 = 8,000
lived in Minnesota 8 months equals 2,000 X 8 = 16,000