Short answer:  Depends on fixed or detachable sight.

The elevation wheel is two parts, a numbered ring, and a threaded round nut which moves the threaded post on the rear sight up and down.  The two rings are held together by the sight body and kept in alignment by an Allen screw.  From the outside these two parts can easily be mistaken for one.  The numbers mean meters but any ring will fit on any nut.  Just won't be accurate.

8/3 numbered ring is used with the M16A2, A3 and early fixed handle M4.  The M16A3 has a fixed handle, of course.  The thread pitch is 1 MOA on the M16A2 and A3.  Shorter sight radius, same sight, on the M4 makes each click about 1-3/4 MOA.

The detachable handle M4A1 (there are no M4A2s or A3s) and M16A4 requres a different thread for the elevation screw because the lift (actually drop) is limited by the top of the receiver.  These use a finer thread that's half the pitch of the standard sight and the numbered ring reads 6/3 to keep the wheel in sync with the elevation.

-- Chuck

The two sights elevate the same distance when you move the wheel from 6/3 to (say) 4.  Because the 6/3 sight thread is finer you're rotating the wheel a larger angular rotation, but the sight lifts the same distance.  If you try to use an 8/3 intended sight with the detachable handle you'll find it sticks out the bottom of the handle and won't mount.

The detachable handle should come with a 6/3 marked wheel.  It's a "half click" sight, with twice as many clicks between 6/3 and 4 as an 8/3 sight.  Should be 6 clicks on your sight between those numbers.

IBSZ will work fine with this rifle.

-- Chuck

As the author of IBSZ I can tell ya that the detachable handle (M4, M4A1, and M16A4) sights have the 6/3 wheel on them.  These handles are Part number 12951011.  The handle assembly includes the sight.

6. Battlesight the rifle per the -10 with the following exceptions: 1. Sight should be at 8/3 -2 clicks, that is, all the way down, not up a click. Please note removable handle sights are marked 6/3 (rather than 8/3); also some are in ‘half-clicks’ as well. There should be 3 clicks between 3 and 4 on the knob. If there are 6 clicks then the sight needs to be set at –4 clicks (instead of –2).

The elevation index wheel has no threads.  Any fit all.  They're available marked 6/3, 8/3, 1...2...etc, and unmarked.  Probably even more variations.  6/3 and 8/3 are "issue."

M4, M4A1, M16A4:
The 6/3 Elevation Index (referred to as Elevating Mechanism in the -23&P) is 1005-01-382-7089.

M16A2, M16A3
The 8/3 Elevation Index (referred to as Index, Elevating in the -23&P) is 1005-01-134-3621.

What the clone makers do is up to them!  Some call their flattops "A3" which can lead to confusion.  The M16A3 (AUTO) has a fixed handle, of course, the M16A4 is detachable.  There are only M4 (BURST) and M4A1 (AUTO) carbines.  There are no "A2" or "A3" carbines.  All but the first very few M4s have the detachable handle.  There was no model change when the detachable handle version was released.  

You'll need TM9-1005-319-23&P Change 7 (or later) to see all the part numbers.  Earlier versions may not include the M16A4.

-- Chuck