Due to the size of the computer, I believe it uses 2.5" HDD and SSD would line-up because nearly all SSD's for mass market now are 2.5" but they are a bit thinner therefore you need a space/frame filler. I know Crucial (in box) supplies the filler.
Note: The following vendor also says it uses 2.5" HDD
When installing 2.5" SSD or HDD in 3.5" storage locations (drive bays), I always use this HP part: 654540-001
I ordered a number of these in 2014/2015 (significantly cheaper at $4 each), before this part became well-known. Shop around (Amazon, eBay, Alibaba) for best price and delivery time (China cheaper, but 3 weeks for shipping).
2.5" SSD to 3.5" SATA Hard Disk Drive HDD Adapter.