One very telling fact is in the English section of the front page of the Guinand website (he took over Guinand until relatively recently when he retired from Sinn itself).

Helmut Sinn himself writes, in the introduction

"I have worked together with Guinand since 1953, when they started producing all Sinn watches up until 1994"

Perhaps this is the origin of the cases? It makes sense in one manner, as Sinn was a German company, but all their watches are Swiss Made

Anyway, the Heuer / Sinn tie-ins that I am aware of are:

The Heuer AMI uses a case that was also used in the Sinn 157 / Bell & Ross Space 2 (as far as we can tell). the Tengler / Arctos / LMC connection I mentioned before would indicate that the case was not in-house at Sinn or Heuer, but came from a third party that ws willing to sell it elsewhere.
Sinn also made a version of the original Heuer Bund (which was later developed into the 156). It was coded the 1550
It was Sinn that disposed of the post-issued Bund Chronos by selling them to the public
One could argue that the original 144 was cased in a Pasadena case, but I have never seen a comparison
Also, is the Sinn 150 case identical to the Heuer 510.403? this one would seem an obvious yes, as it is an almost unique design

Just my 2 cents worth

Cheers
DaveS

PS - I have no idea when it comes to dash-mounted clocks, but both companies have a strong history there and there may be some tie-ups there too?