Can anyone say about SAP PI versions?

Is this 7.31 or 7.4 SAP PI? Or maybe PO?

While working in my organization I found that it shows SAP Process Integration 7.4 but in Software Build Information tab it shows as 7.31. It has both single stack and dual stack features. But as far as I know from few SAP blogs that both 7.31 and 7.4 are versions of SAP PO. Basically SAP PO starts from 7.31.

What concerns single/dual-stack, your SAP PI is dual-stack, also all SAP PI installations up to 7.3 are dual-stack. From 7.5 dual-stack is obsolete and unsupported. Excerpt from SCN:

SAP Process Integration with a release ≤ 7.4 remain mandatory dual-stack systems - so, you can install new dual-stack SAP Process Integration systems with such a release and you can upgrade dual-stack SAP Process Integration systems to 7.4

What concerns PI/PO, you run PI. PO is an integrated product that was introduced with 7.31, though it is not mandatory. You can still install PI standalone, if you do not need BPM/BRM functionality (and that was done by your company).

PO = PI/AEX + BPM + BRM, more on this here.

Moreover, you can even install AEX (core PI component) standalone.

What concerns versions, you have PI 7.4. Don't pay attention to release versions, it can be confusing (like everything in SAP). As stated in note 1515223:

For SAP PI / SAP PO, releases 7.31 and 7.4 do share the same Java code line.

I guess they simply didn't bother to rename the libraries. Consider screenshots from this and this posts, which have similar versions.