Many said that it was the Bronze Milspec. They are partially true. To be more correct and precise, it is a Bronze Milspec, but not an " ordinary " one, as it is, as we'll see, an US NAVY. 

First of all, the funny thing is that Blancpain could have chosen a more famous vintage, such as a Tornek Rayville, or a " regular " Milspec. No, curiously, they opted for a watch which probably never went to production, but which rather was an aborted project, the Bronze Milspec US NAVY. An aborted project, because it seems that the US NAVY didn't accept it. My guess ( a personal guess, nothing less, nothing more ) is that Blancpain offered it circa 1968 to the US NAVY, but they didn't accept it, maybe because they didn't feel the need to get it as they still used the recent TR 900 ( more than 1000 pieces in two batches, in 1964 and 1966 ), or, this is another plausible hypothesis, Blancpain may have charged a bit too much for this watch. 

Some certain facts are that these Bronze Milspec, US Navy or Civilian, are not documented anywhere, and that only a few, very few were made, 45 maximum, including only 4 or 5 US NAVY, versus the 555 pieces of the Act III.  

That being said, let's compare the latest ACT III to see which is the exact one they are referring to... 

1/ The case: 

Same size ( 41,3 mm ) than the Bronze Milspec, US NAVY or Civilian. With these typical lugs. 

The big difference is that the Act III is much thicker, 13, 30 mm high, which may be explained by the modern movement, compared to the vintage ones ( AS 1700 for the Bronze Milspec / AS 1361 for the US NAVY ). 

Act III: 

Vintage: 

So, the case won't help a lot to know which one is the reference for the Act III. They could have gone closer with Bronze instead of Gold, even if chromatically speaking, it is similar.Some will say that Bronze can get some weird patina, at the contact of water... And others will rightly say that the vintage wasn't made of Bronzebut were " Parkerised ". The consequence is that most probably, the Act III is heavier than the original. 

2/ The bezel: 

Same here, the Bronze Milspec and US Navy  share the same bezel, with these big numbers and triangle at 12. Ceramic for the new, bakelite for the vintage, but the same, or almost, glossy look. 

The curious thing is that Blancpain opted for patina for the numbers and triangle on the Act III while, on the vintage, they are greenish. 

3/ The dial and hands:

Now, we'll see where they precisely took their inspiration for the Act III. 

Look at the Act III: 

And now, to the Bronze Milspec: 

And the US NAVY ( Credit Phillips )

The Act III cannot be a Bronze Milspec, as the index at 12 o' clock is lozenge shaped on the vintage, while it is a Triangle on the Act III, exactly like on the US NAVY. 

Second detail: The white printed writing on the US NAVY is conform to the Act III, even if the wording is not the same. The wording on the Bronze Milspec is golden and applied. 

Third detai: The dots are less centered in the US Navy than in the Bronze Milspec, the Act III is closer to the Bronze Milspec, here.

Fourth detail: The hands of the Act III are much more faithful to the US NAVY. Indeed, on the Bronze Milspec, the hands don't have the same shape. The only thing I don't understand is why Blancpain didn't decide to go for an hour hand which could be more faithful to the US NAVY... It is plain in the Act III while it is in two parts, in the US NAVY. But I also saw hour hands without separation, so it might be more a personal preference for me. And as for the indices, they are much more colored in the Act III than in the vintage. 

4/ The crown: 

Smaller and with a round top on the vintages... 

Bigger and flatter on the Act III. 

5/ The movement:  

The big difference between the Act III and the vintage is the presence of a see-through case back on the modern, while the vintage had a solid case back, of course. 

Blancpain certainly opted for a see-through case back to show the rotor of the Act III. 

And here again, we have another hint that Blancpain found the inspiration from the US NAVY, with the Cal AS 1361. 

The Cal AS 1361. Same movement in the TR 900:

The Cal AS 1700 in the " normal " Bronze Milspec. 

So, definitely, we can say that the Act III is a quite faithful re edition of the US NAVY, rather than of the Bronze Milspec. 

I still have to understand why Blancpain opted for the re edition of a watch which was probably not issued. It would have been perhaps more appropriate to go for a TR 900, or for another military Milspec. 

And, if you go for the re edition of a military watch, then a bronze or sandblasted case would have been more à propos, rather than gold, even if the Act III case is made in an alloy of gold, copper, silver and palladium, among other things. 

Last but not least, it would have been more à propos to respect the wording " US NAVY " rather than the writing and wording found in the Bund 2d Generation: 

Or in the early Fifty Fathoms Rotomatic Incabloc: 

Or... In the elusive " AM ": 

All in all, and despite these reserves and inconsistencies, The Act III gathers some indisputable qualities: Finally THE Fifty Fathoms case, no date, and the spirit is here. 

But... At what price! 

Looking forward to reading your comments and thoughts,

Best,

Nicolas