When I look at Fanny Elvira birth and baptism born March 1 1857, see #102, http://www.arkivverket.no/URN:NBN:no-a1450-kb20060216010021 I do see arbeider again with a long extension of letters following it that are not clear. So, her birth record as suggested may support that the "arb" information was transferred from it to her confirmation record
In Fanny Elviras bapt. record the parents are recorded as Ole Paulsen, Arbeidsmand og Martha Jansdatter i Arbeiderboligen paa Hammersborg. I doubt that this has been used for info when the confirmation record was written.