Hi and welcome to RootsChat
The writing on the left says Dup(licate) Star with a medal roll reference and on the right "Deleted Star Returned (under reference) CRV 179/5 of 22/8/1923. Min 28"
Your Gt Grandfather was entered twice for the same medal (1915 Star) under the two references given. The error was spotted and the duplicate medal was not issued to Horace but returned to the issuing authorities. I am not sure what the Min 28 means though.
I confirmed this through the Medal Roll references given which are available on line. So the extra information written in is an administrative notation only rectifying the previous mistake.
The medal rolls give a bit more information that you might not have. Horace is shown as being discharged to Class Z (Army Reserve) on 20/2/1919 in the 1915 Star Roll.
The Medal Roll for his Victory and British War Medals show that not only did he serve with 20th Manchester's as you state but also 3rd, 13th and 9th Battalions. There are no dates given for this Service but he did keep the same Service Number 17358 throughout.
I trust someone will confirm my findings for you but hope that is helpful.
Martin