Rega RB301 Tonearm

Would anybody know the exact effective mass of
the Rega RB301 tonearm?

Thanks so much!

Contact Rega if you want 100% certainty, but my best guess is 11.5g based on the fact that all other specs are identical to an RB300. Check out this spreadsheet at Vinyl Asylum.
As you can see from the above link, all Rega arms are virtually the same in this
regard. Rega themselves, true to form, don't bother specifying this.