You are the lucky owner of a No.1 Bore (.177) BSA Standard Air Rifle from the early 1920's. Your gun has a two hole trigger block, which means it has a non-adjustable trigger pull weight which was set at the BSA Factory in Birmingham at the time of manufacture. (Shortly after your gun was maufactured BSA introduced the three hole trigger block which meant you could adjust the trigger pull weight to make the trigger pull lighter or heavier)
I see your gun has uncommon sling swivels fitted. These were an optional extra which BSA offered at the time of ordering. Alternatively yours could have been added to the gun by a local gunshop or gunsmith after purchase
All in all , you have a gun that is around 90 years old, and in good shootable original condition.