Hymodyne,
I installed the duplex chain on my Mk III engine with no problems but one: No one seems to put timing marks on the sprockets any more.
By temporarily fitting the timing cover and sliding on the timing pulley, you can "dial in" the crankshaft to 0 degrees @ TDC, visually checking the timing mark on the pulley with the timing arrow on the cover.
Rotating the cam to between the lobes on cylinder #1 (As per the Bentley manual) puts both closed, so I then installed the head. Checking the valves with pushrods in place, I was still on the money, so I removed the cover and pulley and installed the new duplex chain.
One thing I noticed about the sprokets on ebay, it may be the angle of the image, but some of the teeth on the larger sprocket appear to be sharp, while others appear to be dull. Since this is a rotating part that you want to install and forget, I'd pay (and did) the extra dollars and get a new one.
-Bob