The Sikh Empire - Religious Images
Depictions of the Sikh Gurus have always been one of the favorite themes in Sikh art. Nishan Sahib banner when they appears in such images are often associated with images of Guru Gobind Singh. As a symbol of royal sovereignty, the Nishan Sahib is usually shown carried by one of the Gurus attendants in a procession with the Guru on horseback.
Two distinct styles of Nishan Sahibs seem to have been in use during this time period. The plain or patterned Nishan Sahib which is a carry-over from the earlier Khalsa era Nishan Sahibs with a similar style and design and Nishan Sahibs with a configuration of weapons as a central emblem.
Cooking Vessel or Dhal Shield
Some scholars [Pashaura Singh, Hew McLeod] have interpreted the round object, which is often depicted just as a solid dark circle with no interior details on some Nishan Sahibs along with a tulwar sword and katar dagger as a cooking vessel, either a bowl or pot. These scholars have theorized that the katar, tulwar and pot must represent the popular Sikh slogan of Deg, Tegh, Fateh - "Victory of the Cooking Vessel and the Sword". 
Conclusive evidence now proves that the round object on these early Nishan Sahibs in not a cooking vessel but a dhal shield. A number of images of Guru Gobind Singh with a Nishan Sahib including two key images found on the exterior and interior of Darbar Sahib reveal details of the round object previously unseen.
On the exterior front face of the upper wall portions of Darbar Sahib there are two gold plated copper panels with human figures. The lower panel features Guru Nanak flanked by companions Bala and Mardana while the upper panel represents Guru Gobind Singh on horseback with attendants and a Nishan Sahib. Like other Nishan Sahibs a configuration with a tulwar sword, a katar dagger and a round object can be seen on this Nishan Sahib. Significantly though, we can also clearly see four prominent metal bosses which act as rivets on a shield face to hold the inner hand straps of a dhal shield visible within the perimeter of the round object.
Inside Darbar Sahib itself is one of the most important Sikh paintings in existence. The only mural depicting human figures within Darbar Sahib appears on the wall behind the northern narrow stairway leading to the top of the shrine. The wall painting depicts Guru Gobind Singh on horseback with attendants and a Nishan Sahib. In the exact same configuration as the gold plated copper panel on the Darbar Sahib exterior we see the placement of a tulwar sword, a dhal shield with its four metal boss rivets clearly visible and a katar dagger.
According to the renowned artist the late Bhai Gian Singh naqqash, this painting was painted by a Kangra artist specially commissioned by Maharaja Ranjit Singh for this purpose. It is said to be a true copy of a miniature painting that originally was in the collection of Raja Sansar Chand of Kangra, which Ranjit Singh had intended to get copied in the form of a mural at Darbar Sahib. Since the artist of the miniature in question had already died, the mural was executed by the painter's grandson. The name of the painter however is not known. 
In addition to the two important depictions of Guru Gobind Singh on the exterior and interior of Darbar Sahib that clearly show the four metal boss rivets that hold the inner handle of the dhal shield in place, we also have other similar images with shield details clearly visible including a gold plated copper panels at Gurdwara Baba Atal and a painting of the Nishan Sahib at Takht Sri Hazoor Sahib in Patna.
1. Pashaura Singh, Khalsa Insignia & Nishan Sahib, www.esikhs.com/articles/khalsa_insignia.htm, 2003
2. Kanwarjit Singh Kang, Punjab Art and Culture, Delhi, 1988, pg. 60