Final scene - Alf's mother's tattoo

Did anyone else notice that in the very last scene, when Alfie and his mum are hugging, that she very visibly displays a 'barcode' tattoo on her upper back?

Given that we only see her in two scenes throughout the entire series, I can't figure out what the significance is?

And if it's a genuine tat that the actress has on her body, rather than a fake tat - why didn't they cover it with a shirt with a higher neckline, so that I don't have to waste time wondering about this sort of thing?

It was a brilliant series, but this sort of detail confuses me.