Recently bought a project hw57 as I've not felt like tinkering for quite some time due to work and health. It's got a broken spring, missing screws, cracked stock and the pop up comes out. Otherwise for a 2004 model it's in good condition with little to no rust.

I'm gradually buying all the bits I need but can't find any info on what the o ring size is on the bottom of the pop up breach. I see they sell them on eblag, but I'm refusing to pay £5 for 4 tiny orings.
Can anyone help?
Thanks
Matt