This is not just S/370 assembler, the real ancient parts are IBM 7074. See for example https://books.google.ca/books?id=3iEDAAAAMBAJ&pg=PA88&lpg=PA88&dq=%22IBM+

This is not just S/370 assembler, the real ancient parts are IBM 7074. See for example https://books.google.ca/books?id=3iEDAAAAMBAJ&pg=PA88&lpg=PA88&dq=%22IBM+7074%22+%22irs%22&source=bl&ots=A_DZmgVBL7&sig=2cAXb5oG_JypmSU_jCa5fqbPqaY&hl=en&sa=X&ved=0ahUKEwjpvuvM2qbZAhVGwWMKHdbXA0YQ6AEINjAD#v=onepage&q=%22IBM%207074%22%20%22irs%22&f=false https://books.google.ca/books?id=3iEDAAAAMBAJ&pg=PA88&lpg=PA... but I read some protocol docs that some of the data is still encoded as it was in the 7074.