Old Boys were due to host Market Rasen Town, but were awarded a walkover after the visitors pulled out of the tie.
In the final they will face Cleethorpes Town Reserves who beat Immingham Town Reserves 5-3.
* Back in the East Lincs Combination, Sutton Rovers moved to within three points of top spot in Division Two.
Rovers scored a goal in each half to beat Alford Town 2-0, through Harry Chambers and Jack Lane.
The win kept them in second place, three points behind the cup-tied Susie Star, but having played two games more, while Alford stayed fourth.
Alford Town Reserves picked up their first point in the Tom Sandys Trophy after holding Louth Old Boys Reds to a 3-3 draw.
Adam Le’gate, Tom Nunn and substitute Sonny Forde all found the net in the Group A fixture, but it was not enough for a win as Louth stayed second.
Also in Group A, Louth Old Boys Vets went down 3-2 to North Somercotes United Reserves.
Carl Graves and Samuel Hallett were on target for the hosts.
Fixtures (Saturday, 2.15pm) –
Division One: Louth Old Boys v Scamblesby.
Division Two: Louth Old Boys Reds v Manby; Sutton Rovers v Susie Star.
Division Three: East Coast Blades v Louth Od Boys Vets.
Tom Sandys Trophy: North Somercotes United Res v Alford Town Res.