83502-37 Large Gas and Oil Tank..........................WLDR
Approximately 4 1/4 gal gas and 1 3/4 gal oil. Can be used with or without Aluminum Oil Tank.

The info above comes from Competition Parts Sheets dated May 1, 1940. These tanks were offered as an option on1937-40 WLDRs.
Similar type of large tanks continued to be offered for later racing models, but were different. They had flat internal flat sides [similar to narrow racing tanks but much larger] and did not have all the oil fittings under the oil tank. They had to be used with under seat type aluminium oil tanks which came in two sub-types: early small fill and later large fill.

Note the later type 1947+ emblems on the tank below are not correct.