Here's some brief analysis of the original straps on the Part 3 mask.
As far as I can see, they were almost certainly 22 mm braided elastic bands. This is an odd width (roughly 7/8 inches), typically used for watch bands. The braids can be counted in the top strap of the Part 4 mask and the side strap of the Part 3 mask in the photos below; in both cases, there appear to be 11 braids in each strap.
The only 22 mm braided strapping I've found has 12 braids per strap, so it is possible they are 23 mm, (even more rare), but more likely just a slightly different braid width.
Using a ruler, I lined up the centimeters in the Dario measurement photo of the Part 4 mask with those on a real ruler and came to a width measurement of 21 mm for the straps on the part 4 mask. Allowing for a tiny margin of error resulting from the tape being lifted off the strap, these almost certainly imply 22 mm nylon.
It is very difficult to find accurate straps for these masks, but they do occasionally pop up on Amazon, often from Chinese sellers.
