These are G Scale Styrene Ladders from Plastruct.

FEATURES: Precision extruded in white styrene plastic.
Excellent for kit-bashing.
Cuts easily for desired length.
Use as a straight ladder

INCLUDES: (2) Styrene Ladders

SPECS: Scale: G 1:24
Height: 1/2" (12.7mm)
Length: 15" (375mm)
Width: 3/4" (19.1mm)
Thickness: 2/16" (1mm)
Plastruct
Part PLS90675