No need to apologise Will, your description of the marking and in particular the location being in the shoulder made it clear it was one of these typically off-the-peg tunics made available to officers through the War Department as I mentioned earlier. And clearly not "put there by the recipient" 😉 It's a gorgeous find and all the rarer for it.
Do you have any other shots of the cuff rank? From the first shot it looks like the owner may have been promoted from Lieutenant to Captain, hence one mismatched "pip" where he had the rank updated. The marks of sewing on the left cuff are likely from where a "Wounded Stripe" was once fixed as this is the exact location they should be worn, eg:
Army Order 204. 6/7/16
...the following distinctions in dress will be worn on the service dress jacket by all officers and soldiers who have been wounded in any of the campaigns since 4th August, 1914:-
'Strips of gold Russia braid, No.1, two inches in length, sewn perpendicularly on the left sleeve of the jacket to mark each occasion on which wounded.
In the case of officers, the lower end of the first strip of gold braid will be immediately above the upper point of the flap on cuff...