I just did a search on pinpics with monorail lanyard as the keywords. They show essentially two series.
The first series says there were four, without numbered backstamps. They had no hidden mickey. Then, they re-released them with hidden mickeys and they were stamped 1 of 8 thru 4 of 8. I found no mention of which ones would be considered 5 thru 8 of 8. They include blue, red, green and black.
The second series is stamped one thru 4 of 4 and includes yellow, purple, orange and silver.
Just don't buy this set off ebay...they are so many fakes of this set! I never did the set but there were 8 original.(three different sets) one set was cast lanyard with no hidden mickey, the second was cast lanyard series with hidden mickey and the third was the hidden mickey reproduction with hidden mickey(confusing enough?) This month we will see atleast one new pink monorail from Florida which is supposedly being released at the PIN Festival. There may be other monorails to come, but I haven't seen them yet.
Not according to pinpics, and it's the best site I've been able to find of pin information. What I suspect is that they consider the four without hidden mickeys the other four of the eight, but have no way of stamping them now.
I've got a couple of them. I'd love to find the silver one. The site also says that there is a pink one, but at this point is being manufactured in Asia, and is un-authorized. Unsure if it will actually be released legitimately or not. It does say that there is a pink monorail at WDW, though, but I've never been to WDW to see it.
The official pink monorail was released last weekend at the event. I still would not buy them off ebay, there are way too many scrappers going around. I believe there are more to come later in the year and into next year.