In 1930 and 1931 the Lefever Double Barrel Trap Gun had double triggers. From the April 1930 issue of
Field & Stream --
Field & Stream, April 1930, New Double Barrel Trap Gun.jpeg
Lefever Double Trap, Field & Stream, April 1930.jpeg
There were Lefever Arms Co., Inc. posters dated 1930; Oct. 1, 1930 and 1931 where it is called "The Latest Lefever Double Barrel Trap Gun" with a list price of $60.
1930 - Oct. 1, 1930 Lefever Poster.jpg
The next Lefever Arms Co., Inc. poster I know of is the Single Trigger poster from mid-1932 when they introduced their Non-Selective Single Trigger. That is the first poster where the Model numbers appear and the Double Trap Gun is dubbed the Model 4. The list price with the Non-Selective Single Trigger increased to $64.80. The Single Trigger Poster also includes the line "Automatic Ejector, if wanted, $10.90 extra." Also, the Hawkins pad is replaced by the new Ithaca "bridge trestle" pad.
By 1934, they appear desperate to move some of these Model 4 Double Trap Guns out and dropped the price to $43.20.
Lefever Model 4 Double Trap.jpg
By 1935, the Model 4 is gone from the Lefever Arms Co. paper and the new Model 5 and Model 6 Skeet Gun are added.
The DGJ reference in the link Frank Posted should be Spring
2013.