Hi Larry.
From the IBS[B/] rulebook:
"The action will have a magazine cut in its bottom and the magazine shall be capable of holding two or
more loaded rounds of ammunition for which the rifle barrel is chambered."
In general, an IBS legal gun needs to have a moveable follower and a cutout area in the stock. It doesn't actually require a metal mag box or spring. Most use a single shot adapter with a piece of foam rubber glued to the bottom of it into the cutout area of the stock.
From the NBRSA rulebook:
"Magazines shall not be required, but actions must be cut
out for a magazine. Such cutout must be large enough to
pass the cartridge for which the rifle is chambered. “Cartridge”
for this purpose shall mean the cartridge as ordinarily
configured, including a seated bullet, when held parallel to the
bore. A cartridge follower or ramp may be installed, but must
be attached to the stock and may not be attached to the action
by any means."
The stock need not have a mag cutout and the follower need not move. Some glue the follower to the stock, others set up their guns IBS-style so they can shoot both sanctioning bodies and be legal for both.
Hope this helps. -Al