Q. - Sequentially numbering rows

Q. Sequentially numbering rows

Advice needed on how to sequentially number rows. - I want to start the second row with 001 and the sequential number to 011. I would then want a space. Then sequentially number from 100 to 160 and then a space then sequentially number 200 to 240. I will also want to add the occasional extra row and add an A to the row above - like this - the new row bellow 105 would be 105A. Seems like a basic question but cannot find answer by searching.