O'Connor's Beretta 450 with a slim BTFE
http://www.16ga.com/forum/viewtopic.php?t=16639&sid=b40ecc4fb31edbb3d1bbac12f76450d7

No mention of a 'beavertail forend' in the 1907 edition of Greener's "The Gun"

Tried a google search and still can't find the first use of the term, or by whom.