This is just an exercise in identification. I have a few tools at home, and some online to help out. All of the items below are viewed from the Front of the stamp.
This cover is from The Commercial Banking Limited Company of Sydney to The Standard Bank of South Africa (located in New York). I purchased it because I thought there was a chance of Die II on cover, but it is clearly Die I. Here is the cover and a closeup of the stamp:
When held up to the light, I couldn't figure out the watermark. I also wanted to leave it on the cover. Using my Avervision with backlit option, I was able to see the stamp a little more clearly (the lines going from left to upper right are from the envelope):
It still wasn't clear enough, so I used Retroreveal to pull out more details:
What we see is a marginal line going down just to the left of the middle. On the left we see some marginal letters (probably an "O"), and on the right we see an upright 3rd watermark, making this Scott 47, ACSC 13A. I was hoping for something different, but I didn't see any varieties or any other distinguishing features.