Old Sheffield Transport Department Bus Driver's Mirror?

[Guest Raisin]

Hi,

I'm hoping that someone will be able to help me - I'm trying to identify exactly what this is...

I bought this mirror off ebay (photos attached) to use as a compact in my handbag (I'd lost my previous one and this one caught my eye) and when I bought it the seller asked me if I knew what it was, he had had it off a chap in Sheffield who had told him it was a bus driver's mirror from the STD (Sheffield Transport Department), but he couldn't work out why a bus driver might need a mirror. I have no idea either - I thought it looked nice in its leather pouch, but thought the internet will know! Which has brought me to you... I'm really hoping someone will know what this mirror was for to assuage my curiosity and that of the chap I bought it off (fingers crossed!).

Thank you and hopefully I've posted this on the correct board!