That seems large for Lantern Plates. My 1890's Ilford Manual of Photography says they are normally 3 1/4 inches square and Special Lantern Plates were "Intoduced to the public in the winter of 1890 (1 shilling per dozen - "Other sizes to order in a few days." )...
As said, it seems they are treated much as contact paper. The manual suggests ID-16 (ID-17 for warm tones). Also says that: "any good metol-hydroquinone developer may be used, including such as are normally used for negatives or bromide paper". " Apparently they are very slow and a yellow safelight is considered suitable.
Have fun, Bob.