does anyone know how to change letter pitch (spacing) modes on an old IBM personal wheelwriter typewriter?

Just what it says on the tin, there :)

Using one of these great, if clunky machines at work - tapping out book spine sticky labels in a library. Far quicker easier and more precise (and oddly satisfying) when compared to trying to laser print them with a computer. But... occasionally there's the odd Dewey code that's a honking great monster and won't fit on the label, or we have some books with impossibly thin spines... we need 12 CPI mode!

At another site, there's a much more spindly Brother machine doing a similar job that has the 10/12/15 spacing mode controls clearly marked: This IBM has the same capability but the controls are uber cryptic. And we can forget about having a manual for it - as far as I can tell, the typewriter was manufactured before the library was built and there's no way of justifying buying a 17.99 replacement manual just to get one keycode. I've tried everything likely and a few unlikely ones, no joy.

Old tech users of the world, come to our aid! :-)