I would say a custom build also-mainly because the port area is way to small for that size woofer.
It will end up being choked off at even moderate levels so the box will behave more like a sealed box.
Maybe more of somebody buidling something they think looks cool-rather than something that actually works.
Those speakers look like they could be quite old, "V" loading has been around since the 1970s.
Old 18's often have less than 1/3 the excursion and power handling as modern woofers, and therefore require only a fraction of the port area.
Used within the power level appropriate for many old 3-5 mm Xmax 18" (and most cheap current 18"), those ports would not have excessive velocity.
The port area is consistent with many manufacturers using speakers with moderate Xmax, similar in size to the first "old school" sub that came to mind.
If anything, the Meyers HP 650-R2 (and many other old "pro" designs) has smaller ports.
I do agree they probably are a custom build.