Hi John,
You must refering to Page 16 of the Stuart, U.S. Light tanks in Action book put out by Squadron/Signal Publications, Armor No. 18 book.
If you have this book and flip over to page 18 you'll find some other Stuart without the sponson .30 cal's and the front fenders completely removed.
The reason why the .30's were removed was for more stores. Plus the sponson .30's were useless on the battlefield. The fender were removed from the M3's because the Japanese threw heavy logs and metal into the works and jamming the running gears.
This pratice of removing the .30's varied from unit to unit.
The M2A4's only combat use was in Guadalcanal. Same thing with the .30's in the M2A4's, some were left in, some were removed.