Interesting! Do I read "US Special Tank Shop"? Could does Shop refer to the "Shop Number", which Montreal Locomotive assigned to each tank they built?
It rings a bell with the "T10E1 Shop Tractor", an experimental American-built Canal Defence Light (CDL) on M4A1 Sherman tank hull.
This needs further looking into...
Source:
https://www.iwm.org.uk/collections/i...ject/205070699