You're probably right about this is not fully custom. It's probably the same font as the baseline "software", simply edited (removing upper parts of vertical stems and /a spur) but really too small to be identified (at least to me). Could be a lot of thing including Helvetica (not so hard to reproduce it using it). The easiest way would probably to ask directly Črtomir Just.
Here, you could find a list of fonts with spurless /a (same vein as the edited "promana"): Typecache