New Barrel won't fit

[FightDesigner]

I've got an older (hand-me-down from my dad) 870, which I've swapped out hardware on numerous times (mostly for movies). Currently has an 18" barrel, but I've got a cool magazine tube extension off ebay that sticks out a couple inches. Figured a 20" barrel would make more sense for that, so found a cheap one, but it doesn't fit. On closer inspection, the grooves in the back are slightly different. It also seems to be a tighter fit in general, wants to go in at a slight angle down (so it touches the magazine extension instead of paralleling it). Are there different barrel types for different 870s? This was an old duck gun originally, not a Police or anything.
Thanks!
Pictures attached.

[Germansheperd]

Aftermarket non-Remington brand barrel?

[Scorpion8]

Remington changed the barrel fit from older 870's to the newer models, but the exact year escapes me. Call Remington customer service with your s/n and they can tell you what barrel fits.

[Germansheperd]

Remington changed the barrel fit from older 870's to the newer models, but the exact year escapes me. Call Remington customer service with your s/n and they can tell you what barrel fits.

they changed them in 1990 but the only difference to my knowledge was the barrel stand off. Ive never had any function issues flip-flopping barrels of any year.