Van Briggle bowl; know very little ! help

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Large clam shaped bowl 12 inchesX14 inches, sits on three "feet" with the bown 41/2 " high. The bowl is itself a clam shell decorated with a mermaid at the hinged end of the shell .her hair falls back to become the ridges of the clam.Her body is laid around on edge. Inside the bowl is a flying fish in relief about 51/2" long. The glazing is very skillful in a matt green accentuated on the outer surface fading to a light green at the base. The colour is superb not truly green but more turquoise. the transition to the lighter colour is speckled but very even. Te inside is coloured in a band 2'' wide the colour is the same as the upper outer surface is the same. The internal band of colour stops quickly to give way to a creamy even colour. the fish motif with its flying fins pass into the colour band the body of the fish is the cream of the remainder of the bowl. However there is a faintly raised band concentric to the rim which is slightly coloured with the green . It gives a very realistic impression of what the inside of a clam shell looks like. There is a brownish coloured ring very much at the internal'corner' of the shell where in life the muscle attaches. there is a more distinct brown ring that may be a stain or water mark but it is not obviously so. We collect pottery but have never had a piece of Van Brigle before. We have no idea of its age but suspect that is 1920 to 1930 based on the marking. Plinted in capitals is" VAN BRIGGLE Colo SPGS "there is no other letter or number. We would appreciate any information and valuation