Got home and found my recumbent had fallen over so before I put it straight I measured underneath. I think I have similar measurements to you.
As I understand the lowrider attachment points number three. Two at the pivot points either side and another underneath the frame to the front. The lowrider attaches inside the rear rack attachment points. The rear rack has a length of tube welded to the struts which is over 1cm each side.
No, the lowrider attaches outside the rear rack attachment. Nevertheless:
The distance between the inside of the rear rack attachment at the pivot point is 120mm.
Agreed, precisely 120mm.
The tube at the ends of the rear rack struts is about 24mm long on one side and 23.4 mm on the other.
I measure (with a steel tape measure, as my digital caliper doesn't go this wide) 167mm between the inner faces of the lowrider (and therefore the outer ends of the rear rack mounting point tubes) at this point. Which would appear to add up.
If you draw a line between these points and measure from it at 90 degrees from the centre to the front lowrider attachment point is 168mm (it's not an exact measurement of fixed points so I not very good at committing to a single distance). If you measure the diagonal to the front point from one side point is 180mm.
Is that the same as your measurements? If so the rack you have should fit mine so I could buy safely from HPVelotechnik.
I'm struggling to agree with you on these. It's not particularly easy to measure this with the rack in place, but:
Eyeballing the distance from the centre of the line between the two suspension pivot/rear rack attachment bolts to the front mounting braze-on on the surface of the frame is something like 185-190mm. I've got a rear rack to hold my ruler against, so I'm reasonably confident about being parallel. Error of maybe 3-4mm or so because the rack blocks my view of the braze-on.
Diagonal from front point to the inside of the rear rack mount (which isn't where the lowrider sits, but is what you've measured above) requires a measuring device that can phase through solid matter, but I'd estimate it at about 200mm. The diagonal from the inside of the side mounting point on the lowrider (sits against the outside of the rear rack attachment) to the front mounting braze-on on the frame is (again, solid matter) something like 210mm. (The lowrider doesn't sit against the frame directly, there's an appropriately curved washer to spread the load that's about 3mm thick).
What I'm confused about is how our measurements are different, as rack notwithstanding, it's supposed to be the same bike.
For shits and giggles, I've measured 135mm from the front lowrider braze-on to the hole (dynamo cable routing, for the purpose of) on the underside of the frame just in front of the suspension pivot. 25mm from the front lowrider braze-on to the rear water bottle braze-on. Eyeball it at something like 190mm from the front lowrider braze-on to the centre of the suspension pivot tube (hard to define the centre by eye). I measured 190mm from the rear lowrider mounting bolt to the centre of the front cross piece of the lowrider. 125mmish centre to centre from the suspension pivot to the bolt on the frame end of the shock. 150mmish (very ish because parallax) from the frame end of the shock to the front lowrider braze-on.
Hope that this sheds light, rather than mud
ETA: Hang on, I've got an idea...
http://www.ductilebiscuit.net/stuff/SMGT_lowrider_side.PDFI've traced around the side of the lowrider: Thickness of the pen means the tube diameters are a bit off, but should give you a decent reference of the shape. Barakta has used her PDF-fu to ensure that that should print at 1:1 scale if you don't tell it to fit to page.
Don't read anything into the slight shadow of the rear mounting bolt head on the paper, it's not quite in the right place (it slipped), the dot is.