I bought a Colt 4 Position M4 stock set from the EE several years ago.  There are no marks identifying the pieces. It works great for me and I am happy with it.  The difference in the extension seems to be the space between the first locking hole and the second locking hole is 1.5 inches and the next holes are 3/4 inch spacing. I assume this may be an early M4 design prior to the current 6 position design.  Does any one know if this is correct.

Best part of this is I just picked up a similar extension from Ebay for about $25.00.  I just bought mine yesterday and have no connection javascript:; the seller.  There are pictures of the extension which is identical to the EE extension I purchased.

4 Pos Ext

The M4 receiver extension is four positions.  

Five and six position extensions are commercial developments.

~Augee

Thanks Augee.  I thought my issue was 4 position.  I must be wrong.