Unidentified fruit

At the orchard where I was picking apples there was a plant (bush) growing through the fence from the neighbours' property. On this plant was a yellow coloured fruit, about the size of a mango and with a waxy peel. We couldn't for the life of us figure out what it was. Can anyone help us out? We don't think it was ripe when we picked one, because it was very hard. Check out the photos and if anyone can shed any light on the mystery fruit's identity, a team of apple pickers would be very grateful!