First, I've neer heard of a GI Longslide. The government certainly did not issue such. It that a particular brand's model designation?I picked up a used GI Longslide recently and this “peening” didn’t show up in the pics....
The peening is evidence of assembly instead of fitting. Every surface that touches another at any point in the cycle of operation may need attention t oensure correct mating and prevention of wear/peening. Yours obviously did not get that attention, just like many more that I've seen. It is nothing that cannot be corrected assuming that the parts are of good quality (proper dimensions and hardness). Get it fixed and don't loose any more sleep over it.
By the way, very attractive gun.