Oh you're referring to a barrel 'extender' not a barrel "extension" right? Blowback 9mm barrels don't use a barrel extension. It looks like it should work without an 'extender', will be close but appears like it should clear.
I miss my S&W 657 Classic Hunter. I had it for several years. One of just a handful of guns I regret getting rid of. I had a silver Leopold FX-II 2x20mm handgun scope mounted on it. Took a doe with it. It was a sweet shooter with that 6.5" bbl.
Ruger has their new Gen II American Predator up on site. From what I can tell it's the same as the standard Gen II American but with a 22" barrel instead of a 20" and has new colors.
https://ruger.com/products/americanRiflePredatorGenII/models.html?n=bolt
I don't have the 10mm or an Elite, just the OG XD-M in 9mm and the rear rails are indeed poly. The rear rails are just a guide, the front rails are integral to the beefy locking block and takes the blunt force when fired. Have had it many many years and unknown rounds through it without issue...