I checked the rear sight off my 240 and there were two of those tiny little buggers in there. Like you said, it will probably function just fine with only one in it since the 240 is such a mild shooting gun. But if it was on anything with more power/recoil I'd definitely want both of them in there. FWIW, I measured one and came up with 0.098".
I don't envy you having to search thru the boxes and packing for that tiny little thing. Been there, done that with those little safety balls in the Crosman pistols. It's no fun searching for something that tiny.
Here's a tip in case you can't find the other one. Several years ago a friend had brought me a click adjustable sight off a powder burner that was missing the ball. I found some tiny ball bearings that were the right size at a hobby shop that had a well stocked RC car section. So if you have a good hobby shop nearby it might be worth a trip to see if they have any the right size.
I still have some of those little bearings but, unfortunately, they only measured 0.093". Not sure how well they would work but if you can't find any the right size I'd be more than happy to send you a couple of them. Just let me know.
Oh, yeah. Your 240 is sounding good so far, other than the snafu with the rear sight. Look forward to reading how it does when you get the chance to do some shootin' with it. Hope it shoots as good as mine does. Also look forward to hearing what you think of the T05 when it's matched up with that lighter mainspring.
"You've come far, pilgrim."-------"Feels like far."
"Were it worth the trouble?"-----"Ah, what trouble?"