Yeah it was something of a mystery to me before too
About the sources:
Firstly the book - Suomalaiset Panssarivaunut 1918-1997 (toinen laitos 2003)
Secondly I eyeballed the Vickers 6-ton and the gun tank converted OT-133 at the parola armored museum last summer. And surely enough both had the fourth crew man whit a bow mg mount (suomi for vickers and DT for OT-133).
