(Reuters) -Striking Boeing workers on the U.S. West Coast will vote on an improved contract offer on Monday, which includes a 38% pay rise over four years and a bigger signing bonus, their union said on Thursday.

More than 30,000 factory workers who produce Boeing's ( BA ) strongest-selling 737 MAX commercial jet and other planes have been on strike since Sept. 13 and have rejected two earlier offers from Boeing ( BA ).

Boeing's ( BA ) latest offer includes a $12,000 ratification bonus and the reinstatement of an annual bonus scheme that was not included in a previous offer, The International Association of Machinists and Aerospace Workers said in a statement.

IAM said it was endorsing the offer ahead of the vote.

Workers have been pushing for a 40% wage increase and the return of a defined-benefit pension that they lost a decade ago.

Last week, some 64% of the planemaker's U.S. West Coast factory workers rejected an offer of a 35% general wage increase over four years that was not endorsed by the union.

Boeing ( BA ) shares rose 2.5% in after-hours trading after the offer was announced following an exclusive report by Reuters. Shares had closed down 3.2% earlier on Thursday.