Swedish icons Opeth have posted the following message via social media:

For many years Martin (Mendez) has been playing Sandberg Guitars basses both in the studio and live. Based on Sandberg California TT Passive, the Martin Mendez Signature features a European alder body with Soft Age finish in Virgin White or a European ash body with a Black Matt finish. The 6-bolt-on Canadian hardrock maple neck with matching headstock and the ebony fingerboard with 22 frets provide a nice bite and a fast response. 

The Black Matt model also offers a maple fingerboard with black block inlays. The Martin Mendez Bass is available as a 34”- scale 4-string and as a 5-string with 35”-scale.

With the black Sandberg hardware for improved sustain and bolder tone, and the black lightweight tuners for perfect tuning stability, the Martin Mendez Signature appears unpretentious but classy. 

As Martin wanted a classic and traditional sound with a certain bite he, chose two Delano JMVC pickups set in a special position that is a little bit closer to the bridge than Sandberg´s Standard TT position. The tone one can be further refined and tweaked with a volume control, a balance control and a treble cut. Finally, the Martin Mendez Signature Bass is subjected to the Sandberg 'Vibration Treatment', which results an improved resonance and an authentic old school sound of an instrument that has been played for many years." 

Go to the Sandberg Guitars page here for more details.