Hi Simon,
I didn't do too much research into the holes for the article but did spend time on this for the U-Brass set, which included a drill template for these holes.
I think 120mm is a wee bit too large. I can't say for sure because we just working from photos, sometimes with not great resolution, and often taken at angles against the vents. The latter is a pain as it distorts calculations.
When we did the U-Brass set we reckoned that the circular hole was around 75mm. This was worked out by comparing the width of the hole against the space between each hole. This worked out with a few photos at around a factor of 0.4 (or 2.5 doing it the other way).
For example if the space between holes was 180mm then the hole itself would be 72mm.
Its hard to come up with a definitive figure for the holes but I reckon 75mm looks right against photos, whereas 120mm is too big.
Cheers,
Dougie