I think Millie is sitting in a motorbike sidecar. In 1928 Armstead won the six days Motorbike trials with sidecar for Middelsbrough MC (The club still exists and they have the Trophy). The story goes that Millie actual drove the bike that won, but it got offf near the end to let Armstead drive over the finsh line as it was not the done thing for girls to ride motorbikes! Or win races in thse days. This a very rare photo of Millie wearing a dress! She wore trousers! Shock horror!

Umm
the horizons a bit off...

Brilliant bit of history!