The circled "T" mark appeared on M-S rifles around 1920 (when production resumed following WW 1) and denotes the use of a new high grade steel. The mark is understood to have been introduce either during or just after the War. So, that being the case, your rifle couldn't have been made before 1920. However, as noted above, all non-British made rifles imported into the UK were marked as such from 1925,(which your rifle isn't) so that tends to suggest to me that your rifle was probably made between 1920 and 1925.