Commercial CLLE MK I* question

Hello all,

I recently picked up a commercial BSA CLLE Mk I* rifle. On the left side of the barrel, it is marked "303 nitro proved." Below that, there is an "OT" stamp. Does anyone have an idea to its meaning?

On Joe Salters website, he is selling a couple of long lees with commercial barrels. On one of the REALLY spendy ones (in the pictures), the barrel is stamped the same, but has a "BT" stamp.

I am assuming the "E" within a circle denotes enfield rifling.

I appreciate any input.

Arnold