Hola Gus,
Yes the Marcelo' s combo is equivalent to Er20L+Er40L. The new S&S ring series are "L" named 'cause the security lock inside the ring, but they offer the same extension of SXER (20mm) and ER40 (40mm). Sea&Sea also has introduced a new ER30L (30 mm) and finally they offer a secure and varius extension ring system.
In any case don't trust in sea&sea lens chart, it's incomplete and contain mistakes.
I shot with MDX housings and I've tried different options with different lenses, in my experience with rectilinear zoom wide and dome port 240 (old type and new one) I've obtained best results when the front lens is quite complained with the base of the dome port. This empiric regula guarantee clear shots and also improoved using the new corrective lens Sea&Sea M77.
I give you the sets that I have used on dome port 240, most of those are not documentated in s&s lens chart
Dx sigma 8-16 --> ER40: quite good
FX Nikon 17-35/2.8 --> ER20 + ER40: quite good
FX sigma 24/1.4 Art --> M77 & ER40: very good
FX Nikon 18-35/3.5-4.5 G --> M77 & ER20 + ER40: very good
I suppose that Nikon 17-35 can give better results using M77, unfortunately last summer I crashed the lens on rocks (and me too) so I was unable to try it; I've purchased the cheaper Nikon 18-35 G and it surprised me giving great quality pictures BUT NOT BY SEA&SEA LENS CHART INDICATIONS!! S&S suggest to use only the ER40 in combo with M77 lens. I've tried it shooting very orrible pictures: internal dome reflections and poor sharpness in the corner. Great result, thank you S&s. I've added ER20 extension and everything changed: no inner reflections and hight sharpness all over the frame.
In your case I suggest the M77 and Er40+ ER20 , but don't miss to consider ER40+ER30 it could surprise you.
I hope to be usefull
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