H.M.P. mystery canvas wallet

I picked up this small mystery canvas tool wallet because it looked interesting in that it appears to have been made at a U.K. prison for the U.K. MOD.. I have no idea what it's actual intended purpose is other than I assume it is for holding small tools such as rifle cleaning tools or small repair/service tools such as screw drivers etc.

Does anyone know the actual location of HMP 35, please and is anyone able to confirm the correct use for this item, please? I have looked online for the location of HMP 35, but without success. Many thanks for any information and help.