I have such a curious spring/plug keeper on my Remington 11-87. Conversely, I had a much different design on mine old Remington 1100. Oddly, when I disassembled the magazine this morning to install the extension and its longer spring, I found the plastic keeper to have broken during the hunting season. It no longer keeps. Worse, when the spring launched itself, the plug, and the keeper at high velocity unto the ceiling and to wherever they all bounced to, it was only the broken keeper and the spring that I found. The keeper no longer keeps no matter how I turn it. It be just another case of some cheapie plastic manufacturing short-cut gone wrong. I have had similar problems with cars and trucks. I threw the keeper away, and now the end of the spring is up against the bottom of the screw-on cap. It is poised to spring out like a jack-in-the-box when the cap is unscrewed again. I am hoping that the plug will turn up again some day.
It does not strike me that having the keeper in it's half-inch inward position is relevant in as much as the extra compression is minor compared to what it would be if there is one, two, three, or four 2 3/4" shells in the magazine. Hunting birds in Washington, I never had more than two shells in the magazine.
On the continental trap range today, I had six failure to feed incidences out of 25 with the magazine extension. This percentage is slightly less than with it was without the magazine extension, but it might be attributable to luck. I told a bloke there about why I had the extension as I am sure that it looked rather strange for being on the trap range. He replied that he had the very same problem on his Remington trap grade 11-87 with only a 2 3/4-inch chamber. His solution had been to trade the gun in on an over/under. He recommended a particular gunsmith in Tacoma. He also recommended looking into having the gas bleed-off hole reamed out wider. I would be most careful about something like that for obvious reasons. He did say that he bought the Remington 11-87 through that very same man. That is not a discredit to the gunsmith as it is possible that the trade-in happened as an alternative to asking the gunsmith of fix it.