I'm pretty sure Maneuver didn't come out 'til the 90s. I'm not sure how complete the bikepedia is when it comes to the Maneuver, but they list it as 24" wheeled kids bike only up 'til 1996 which came in Dk blue and had a 1" stem. Check the specs against your bike in attempt to confirm, assuming the previous owner didn't strip a lot of parts off and just did the bar swap.

Since I can't imagine Mongoose using a BMX stem past 1985, if they ever did, on a MTB, I think it's safe to say it's a standard 1" stem, or at least the fork is designed for a standard 1" stem.

Either that or your measurement is WAY off for a 1 1/8"...