The standby instruments serve as a backup in case of a defect or for a X-check of the PFD and ND (DU's).

In the MIP of the B737-800 there are three (or two) standby instruments.
Three instruments for the analog version and 2 instruments for the digital version. The first Stby instrument is then covered with a picture.

The newer B737-800 (NG) are more and more equipped with a digital version, i.e. a digital ISFD and a standard RMI.

I have this MIP is equipped with 2 digital standby instruments ISFD and RMI).

The standby instruments are located between the Upper Eicas and the DU’s of the Capt.

More info and photos : http://www.flightdeck737.be/hardware/instruments/standby-instruments/