(ET) Front wheel bearings

[Ken Olum <kdo cosmos5 phy tufts edu>]

Today one of of the front tires on my E-15 got a flat.  "No problem,"
said I, because I have two spare front wheels.  However, when I took
off the wheel, it was not the same as the others.  The one I took off
had two bearings, one on the inboard edge and one on the outboard edge
of the tube through the center of the wheel.  However, my spares had
something set into one end, but not the other, of this tube, and it
didn't show any sign of wanting to turn.  Are there two different
designs, and my spares are not actually compatible with my tractor?
Is there a way to remove the bearings and put them in the other kind
of wheel?

Are the tires some standard size?  I'm worried because mine say
"Elec-trak" in the rubber.

Thanks.

                                Ken Olum