Drum brake wheel cylinder question, which way do the pistons face, D shape behind or in front of the shoes?

Hi there working on my sons Fabia mk1. Mot fail on rear brake imbalance due to leaking wheel cylinder and a front flexi plus an advise on a track rod end so not bad at all for 2006 car. Got new ones and quick question  there are D shaped lip on the end of the pistons. Not sure if they are supposed to be behind the shoes or in front of them.The old ones had a groove that the shoe sits in.

Hope this makes sense

Alasdair