A security device has a flexible strap (26) with a lock unit (28) attached at each end. The strap (26) comprises a plurality of longitudinally extending multi-filament cables or ropes (2) arranged in a substantially planar array embedded in an elastomeric material (4). The cables or ropes (2) have a coating of primer for creating a bond with the elastomeric material (4). The primer may be restricted to the external surface of the cables or ropes (2), or some of the surfaces of the filaments may be free of primer. This facilitates relative movement of the filaments during flexure or compression of the strap (26). Moreover, an extrusion process for manufacturing the strap (26) including a priming station (12) is disclosed.