Thank you for taking an interest in my problem, and for the link to the pdf file on M. Butkus' site. I was already familiar with the file, and while Butkus' list of Sunpak flash instruction manuals certainly is very impressive, it unfortunately does not include the OT220-S model of which I suddenly found myself possessed. Furthermore, none of the listed flashes resemble my own to any helpful degree; for instance, the 220-S offers no film speed selector (cf. the picture in my original posting), while most of the models listed by M. Butkus do.
I think the 220-S must be very old indeed, for I haven't found any mention of it anywhere on the internet.
With the generous help I have had from kind users of this forum I am pretty confident that I shall be succesful in this my first venture into flash photography. Not that I shall make a habit of it, though - it's nice to try something new, and a flash might actually come in handy some day, but I think I shall stick to taking pictures by available light. That offers enough challenges to last a lifetime.
All the best,