As much as I hate replying to my own comment, another five minutes googling reveals: The typewriter you're looking for is the IBM Executive, Model D. It featured variable spacing, superscript characters (as required), and while the model D may not have been in every office, was a simple matter to order and use. It was also customisable in terms of individual keys being replaceable. I also note with interest the various font experts who have explained that the MS Times New Roman font has many differences from the IBM font that was used in the document in question (particularly in the numerals). Have a nice day.