Frank: the first two of your links did not work - presumably they are links to your own photo gallery. There is a little quirk of our website, which comes up from time to time: You need to open a second instance of the JRA website and instead of going to your "view profile" and from there to your "member photo albums" (which are viewable only by you) - instead you need to go to your "My directory profile" and access your photo links from there. The links from "My directory profile" are different, and will lead to results which are viewable to other members.
Clear as mud?
Alternatively, you can go to your "View profile" then to your "member photo album", access the photo and copy and paste it into your post - but (for the benefit of other readers) you need to grab the "handles" and size the photo to about 2/3 of the viewing screen width in order to avoid another quirk of our website (if you don't, the accompanying text will be forced outside of the viewing area.)
Here are Frank's two photographs:
To add to Frank's useful post, may I add:
1. Arne has an aluminium fabricated masthead detail in one of his writings, similar to the de-lux fitting shown above, and to Tapatya's as posted by Kevin.
2, Paul Th has shown a quite different and very practical type of masthead crane (a cross tube) for the masts he makes.
3. The most sophisticated idea on record is the masthead fitting on Ted's Fly which was reported to be engineered to allow rotation, so the halyard spans and lifts can rotate with the sail. "...swivelling cone(s) on acetal bearing(s)..." (Ref JRA magazine #88)
The common factor to all of these is that the masthead blocks are craned out, so they can hang vertically. This seems to be essential and ought be be obvious. I am sure Frank already knows this, but I add this note for the benefit of mistake-prone people like me, and dinghy sailers thinking of converting to junk rig. The first masthead fitting I made was only a for a small boat so I didn't bother about the need for a crane. I just fastened the blocks to the masthead by using saddles attached vertically to the walls of the mast.
It was a failure because the blocks did not hang vertically, which put a lot of friction on the halyard - the problem was immediately obvious and that otherwise nice and simple idea had to be scrapped. I mention this because I observe that I am not the only one who has made this mistake. Note: the saddle shown here is the problem.
Evidently the U-bolts used by David T and shown in the second of the three photos given by Frank (see below), are a different shape and do act as cranes, allowing the blocks to hang vertically. This detail is not quite as simple as it looks.
For a small boat, dinghy etc, the plywood disk type of crane (as shown in the third of Frank's three photos) is a nice and simple detail which is what I settled for, as it works well and you can make it yourself using little U-bolts or saddles. 
It is bolted to a decent plug in the top of the mast, and is fixed against rotation. - simple if the masthead is an untapered tube - likewise simple if the masthead is solid. Perhaps not so suitable for a larger boat, or a tapered masthead tube.
I must say, I am impressed by Annie's ingenious disk crane with webbing tangs
