See also Continuity Guide.
This spacesuit backpack seems to be the same as the one worn by Helena in Space Warp and Kander in The Bringers of Wonder part 1.
The central hole was made for the "emergency oxygen" fitting which appeared in Space Warp. For consistency, all the backpacks had the same modification.
Also in Space Warp, an oxygen dial was added. This has now been lost, but inside you can see blocks of wood giving structural support to the sides.
Page copyright Martin Willey, thanks to James Winch