Putting push rod in brake servo

Hey everyone, hoping someone has any clue as to what to do here.

my brake servo failed recently (vacuum leak), so I went and got another one off an eBay breaker. It arrived and it looks good except for the push rod which I expected to be fixed inside (like my old one) came separately in the box. This means I have to fix it in there somehow. It looks like there’s a seat for it but no way of fixing it in place. Anyone done something like this before? Pics below.

Thanks in advance, really hoping I’m just being and not got a duff part!