Hi Ivica,
M2 waveguide is said to be 120° horizontally, which corresponds to a 15" around 800Hz according to the Altec chart you linked above.
There will always be "problems" on the vertical axis with non coincident drivers, and looking at the power curves the Linkwitz-Riley characteristics are quite obvious around 800Hz :
- the "listening window" measurement tracks the on-axis one, which implies a well behaved forward lob
- the early reflexions curve shows a dimple, which is the result of the off axis null
- the sound power curve shows a ~3dB dimple, which is also expected with a LR crossover
All in all I'd say it looks like a pretty well behaved system
Regarding lens directivity behavior, here is an excerpt of the Smith/Keele/Eargle 1981 AES article on the 4430/4435, with directivity measurement comparisons between expo+lens and 2344 horns: