Crosman 13xx/2289 Forearm Pops Open

My Crosman 2289 Backpacker has developed an ism where the forearm pops open under pressure. I found this out when I laid the rifle across my knees, and suddenly got nicked in the junk by its forearm popping open. I now have a velcro band loop that I slide up the forearm after pumping to keep it in check.

Causes?
Fixes?

Don't open the image if a dead rodent offends you. please and thanks.

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