I don't think that the V in SPIR-V has anything to do with the number 5. I think it's just that SPIR was renamed to SPIR-V with the major version that was introduced alongside the Vulkan graphics API and made SPIR usable for more than just OpenCL.
IIRC somebody said during one of 2015 SIGGRAPH presentations when it was introduced that it's the fifth iteration if you count SPIR pre-1.0, 1.0, 1.1, and 1.2 preceding it. They may have been joking.
Of course, the V in Vulkan also stems from 5. Vulkan was originally going to be OpenGL 5. That plan changed, but they chose a name starting with V as a nod to that history.
I'm sure that the name is alsona nod to Mantle, the direct predecessor. I haven't seen the Mantle documentation, but rumor has ot that Vulkan 1.0 initially was little more than Mantle with renamed identifiers.
