I have heard (but can't verify) that Rigby in California are particularly bad about giving out information on older rifles. However, Paul Roberts at J.Roberts and son in London (former owner of Rigby and now owner of Jeffery) has copies of all the old records and may be able to help. I am guessing from the Mauser S/N you give that it is 1924-25 vintage. Somewhere there should also be a (4-digit) Rigby S/N which may help narrow it down. Others on here who have more Mauser knowledge may be able to give a better indication. Hope that helps.
I forgot to say - a wonderful rifle and heirloom!