"Ultimate" is a big word, but at least this is a very good try at getting to grips once and for all on how Canon lens weather sealing works.
The question is: do your non-white weather sealed L series lenses need a front filter to be rain and dust sealed?
The short answer is: a hand full do, the rest do not.
Long version:
I had a discussion in another thread where I claimed that Canon's non-white weather sealed L series lenses needed a front filter to be rain and dust sealed. These lenses are:
EF 14mm f/2.8L II USM
EF 16-35mm f/2.8L USM +needs front filter
EF 16-35mm f/2.8L II USM +needs front filter
EF 17-40mm f/4L USM +needs front filter
EF 24-70mm f/2.8L USM
EF 24-105mm f/4L IS USM
EF 100mm f/2.8L IS USM Macro
EF 50mm 1.2L USM +needs front filter
EF 85mm 1.2L II USM
EF 24mm 1.4L II USM
Someone disagreed (someone always does) and asked Canon, USA, and got the following reply:
"Only our white "L" Series lenses are weather sealed. None of our weather sealed lenses need a filter to be weather sealed."
Needless to say my jaw dropped in disbelief, as I knew this was wrong. However, I also now that Canon's info on this is sparse and difficult to understand, so I decided to get things right once and for all, so I replied:
"I'll take the opportunity to pose the same question to Canon, Japan, and see what they say... "
And what did Canon, Japan say? Well, they actually also did not get it right the first time I asked (but at least it was ALMOST right). But to spare you the details here are the hard facts in translation:
"The water and dust proof mechanism is explained in the below URL:
http://cweb.canon.jp/ef/technology/eflens-technology.html#tecProof
We do have some water and dust proof Lens, but to enhance the proof, it needs to be used together with water and dust proof camera.
Also, for some Lenses we ask customers to put filters to prevent water coming in from the front part.
These are the Lenses we ask to have filters on:
・EF16-35mm F2.8L USM
・EF16-35mm F2.8L II USM
・EF17-40mm F4L USM
For details, please refer to the below URL:
◆EF16-35mm F2.8L II USM 特長
http://cweb.canon.jp/ef/lineup/wide-zoom/ef16-35-f28lii/features.html
◆EF17-40mm F4L USM 特長
http://cweb.canon.jp/ef/lineup/wide-zoom/ef17-40-f4l/features.html "
As I knew that this was not quite right I wrote to them again:
"I am sorry for our mistake.
We have forgotten to mention EF 50mm F1.2L USM.
As you mentioned, this also needs a filter on the lens."
So for these lenses you need a front filter for the lens to be fully rain and dust sealed (I also noted this on the list above for a quick full reference):
EF16-35mm F2.8L USM
EF16-35mm F2.8L II USM
EF17-40mm F4L USM
EF 50mm F1.2L USM
And remember: The seal at the lens mounts will only work with a select few professional Canon bodies. Canon 5Dii and below need not apply.
Is this then the ultimate truth?
I really hope so, but I'd be lying if I did not admit that to me the front of the 50L and 85L II are such lookalikes that I almost wrote to Canon, Japan a third time...
The 5D was made for the conditions over there too 
