A manual entitled ‘horizontal mill’ is available, which contains some details of the vertical and universal machines.
https://www.ebay.co.uk/itm/285489101918
Look hard and you will find it elsewhere for no charge. It might be a starting point. Photocopy the drawings up to full size based on a known dimension and scale the rest.
There is a Harrison group at groups.io . It is more for the lathes, but its message archive is publically browsable so you could see if there has ever been discussion of milling machines there.
G&M Tools seem to have a bit of NOS Harrison stuff, so you could try them.
Put an advert in the classifieds here, and in the magazine and on homeworkshop.org . It could be like a lonely hearts one: mature milling machine owner, OHAC (own Harrison and calipers), seeks similar for mutual size comparison of interlocking parts.