'MA' on the barrel just means the barrel was made at Lithgowicon, and the 'BA' on the trigger guard I think means it was made at the Orange factory. Bunch of different factories came around later and built components and assembled rifles. If I am not mistaken however the Orange factory was not around in 1916 when your rifle was first built, which makes me think the trigger guard has been replaced. History of factories is not my thing so check with someone else first.