Fork connundrum

[Peanut]

Here's a weird one, I have two forks and one headset, the headset fits both forks perfectly all nuts screw on nice and snugg, I also have one stem which fits fork A perfectly but is way too big to fit into fork B, the guage of the tubing on both forks is indentical.

I'm forked if I know what's going on!

[theRuler]

if the gauge of the tubing is the same then it must fit.

[Peanut]

get this, tried an old se stem and it fits both forks.

it's a damn mystery to me.

[Frosty]

Sounds v odd, but if the internal diameter of the tubing is the same then there must be something in the way?? Star spangled nut??

[gjs]

Or its butted tubing