A refined version of the S-3 with revised strutting carrying the centre section of the upper wing and the root attachments of the centre wing, a modified undercarriage and other changes, the S-6 triplane of 1917 was the first US "scout" to be fitted with twin forward-firing machine guns, these being gas-operated Lewis guns (which were mounted side-by-side on inverted and inclined "V" struts immediately beneath the centre section of the upper wing and firing outside the propeller disc. Only a single example of the S-6 was built and tested.