There's a new version with updates of the MT3 from the same designer who did the original. Even that one doesn't match the "magic" of the original as others have said about countless clones. You can use the internal effects to get something that does the same general function - just like we can emulate different cabs and heads but it just doesn't sound the same as a real Fender Twin (for example).

The first name the designer gave the pedal was "Auto Wah". You'll notice that's a built in effect choice.

Yes, the pedal was an envelope follower and a VC filter. There are even schematics available if you want to study those.

See this thread:
https://www.yamahasynth.com/forum/wah-effect-mystery-parameters#reply-20272

Thanks. I didn't realize that a Mutron III was a kind of wah effect.

It was the ultimate Auto Wah!

I just looked at the thread you directed me to, which was a question I had asked, and I noticed that you had told me in that thread that a Mutron III was an auto wah.

There's so much to remember in the world of computerized music and synthesis. It's way more that I can hold in my head at once. Hopefully I'll remember this time...

From the thread you referenced:

Extra Credit: Look up classic 1970s stomp-box the "Mu-Tron III" which was an Envelope Filter (also referred to as an Envelope Following Filter) this was the ultimate Auto Wah... back-in-the-day! the character of the unit is righteously emulated (imho).

From your description in that thread of the three wah effects, it doesn't appear to me that anything like the Montage Envelope Follower is involved. Am I correct in that?

You can certainly construct an Envelope Follower to do an auto Wah by directing it in the Control Assign to "Cutoff". The Envelope Follower can be assigned to any number of parameters. You can use an ARP to create rhythmic pulse (a Kick drum can be the source, set the Part Output to Off). You can shape the application of the filter sweep using the "Curve" and "Ratio" parameters.

Auto Wah can be setup via a Motion Sequence.
You can even create an auto Wah as an Arpeggio... Controller Arps can send cc74 and cc71

Fundamentally, a Wah pedal is a filter sweeping device. The grit, overdrive, noise, "hair" of a classic Cry Baby or Morley pedal was part of the charm. You'll need to add all that character with other processing, if you miss it. The Filters in Montage don't provide grit, overdrive, noise or "hair", but there are plenty of ways to add that stuff.

You can use either the Envelope Follower or Motion Sequencer or Arpeggio to create all kinds of Automatic Filter control... that is fundamentally what auto Wah is about.

You can fashion the Envelope Follower to be a "Touch" Wah, using the Key-on velocity of one Part to control Filter movement in other Parts... totally doable...

But, imho. The VCM "Auto Wah", "Touch Wah" and "Pedal Wah" Insert Effects are the best modeling I've heard of being able to capture the feel of the classic Mu-Tron III (something I personally used back in the day). like any Effect, you have to sit with it and play with the settings. Once you understand the "Wah" as a filter sweep, that can be allowing more highs or working the opposite direction, taking away lows. So the initial Center or Cutoff Frequency is a key point. You don't filter all of the signal - so the position concerning unfiltered signal (the Cutoff Freq itself at rest) is going to be critical. (So yes, a Wah pedal actually does a poor job of filtering, but that the point, that's the sound). An absolute accurate filter would do the "Wah" thing but it won't feel the same. Better is not always best.

The "Touch Wah" is particularly suited (again, personal preference) for emulating a Wah for keyboards, as in Steveland Morris' "Higher Ground" (classic Mu-Tron III used on that clavi part). Instead of a steady rhythmic (Auto) Wah that is following a clocked LFO or rhythmic source, the "Wah" or Filter movement is attached to the input velocity created by each Note-On. This way the Wah is on each key press and is dynamically controlled by your key-on velocity.

You mentioned you were going after the Jerry Garcia use of the Mu-Tron III... Guitar player approach can be different... Again, I believe the "Touch Wah" is going to be on-point.
Just to complete the the subject, among the Insertion Effect Types you have a "Dynamic Filter" to explore. Like the "Dynamic Phaser" we introduced in the tutorial a few weeks ago to create those Phaser "bubbles", the reaction of this Filter is based on the signal strength. Something to explore!