Oil Tank for a ? ? ?

I found this on e-bay. Don't know how to get the pictures on FHP website.
This is an oil tank for a BT, 6v, but I've never seen one like it before. Could it be a Police bike ??
I've looked in all of my books, can't find any thing that looks like it. Found a BT Flat Head that looks close,
but it's not the same.
Pooka
" Harley Davidson oil tank, and battery bracket from the early years. Good shape, and very usable for almost any bike. Definately in need of a new paint job." Item number: 160187106587

Here it is Pooka.
JKE

Yup, That is the tank for the police Big Battery unit. Started in the early '50's like this one and later on had a built in oil filter.

Yup, thought it might be a police bike oil tank. Thanks every one !

I found this on Panhead Flathead site . Cop bike with the big batt. and side oil tank.
1956 FL Police .