There is a buffer ring that goes INSIDE the slide extension assembly, which is what is pictured. If the buffer is in place, the assembly will leave about a 1/8 gap between itself and the receiver when pushed all the way towards the receiver. It will otherwise contact - and make contact upon recoil - with the receiver. DO NOT shoot the gun if this is the case. You WILL bend the extension assembly and then good luck finding another. Had that happen to me and it took me a year to get a replacement part. If the assembly does not freely slide back and forth with the the prongs sliding without resistance in and out of their corresponding holes in the receiver, the damage is done. On mine the prongs ended up bent upwards just a tad creating a force fit. The buffers are cheap and still readily available on eBay.
