I use the shortest words possible when creating fields in a custom list. I go back and rename it sometimes, but the URL retains the shorter name.
I have created a list, and have fields IS1 thru IS10. If I name the column I or IS, the internal field name matches, but when I add the number, it changes the I.
This is what I see for IS5: Field=%5Fx0049%5FS5
I also created a field for I and IS for comparison:
Can anyone explain why it would do this?