Probably obvious, but have you tried mailing Digigram and asking if they could provide a link?
Some companies basically ignore the existence of their older products and tell you you’re on your own even if they do have the requested data. Others even go as far as to provide schematics and service manuals to their 50 year old devices on request (thank you Dynacord). It might be worth finding out which camp this particular company belongs to, if you haven’t already.