I only just started a research inquiry about Tuttle & Clark and the first thing I found was an 1895 ad for horse harnesses, horse blankets, saddles, etc. from Tuttle & Clark in Detroit. I will keep digging, but it seems that may be how they started.
In 1897 they advertised their "fine fur robes" (lap robes or blankets).
In 1899 Tuttle & Clark opened their new Detroit horse-goods store.
In 1910 Tuttle & Clark advertised their driving caps, gauntlet gloves and waterproof automobile dusters (coats). I guess they saw the writing on the wall with the popularity of automobile travel.