Murex is the ultimate modern fountain pen. You can carry it everywhere, travel with it, sit on it, toss it in your bag, knock it around, and it never fails you. This is the pen you'll want to grab as you go out the door. And this is the pen most likely to grab everyone's attention when you whip it out to jot a note.
The first two short models were actually named "MYU" by Pilot. The later models full size models were called Murex. These were two distinct pen lines, although related. While western pen collectors lump both of these together under the Murex banner, most Japanese collectors do just the opposite and refer to both groups as "MYU" pens, short MYUs and long MYUs.
Pilot MYUs and Murexes were only sold here in Japan, intended for the Japanese market, which explains why they are so highly valued in the west. Even in Japan they are difficult to find since Pilot no longer makes them. But they are such tough pens, the ones that do turn up are usually still in great condition. These are workhorse pens.

Seeing their popularity today, it's an amazing fact that Pilot stopped making these pens over twenty years ago. I asked Pilot pen craftsman Mr. Hirosawa if they ever considered making these pens again. He acknowledged their popularity right now but smiled and shook his head "no" at the suggestion.

They are often compared to the titanium Parker T-1 pen which was introduced in 1970 just before the Pilot MYU came out, and was produced for only one year.
The first Murex appeared in 1971, and was called the "MYU 701" in Japanese and identified by the Greek letter for "m" in Pilot's advertisements. Its model number is M-350SS. This pen is displayed in the Pilot Pen Museum (now called "Pen Station") in Tokyo. This one of those ingenious Japanese short style pens with long cap and short barrel that made the pen short when capped, and full size when the cap was posted. It was an extremely streamlined design with no markings anywhere except for the small Pilot name engraved on the cap. There is also a tiny date stamp with month and year on the barrel.

This model is considered the quintessential MYU/Murex, and therefore the most popular model today. It has also been called the "ultimate travel pen" because of its small carrying size as well as its sturdiness and reliability. It retailed for 3,500 yen when it first came out over 30 years ago, but an original MYU with price sticker (identical to the one in the photo) was sold last year on an internet auction for over 500 dollars. The market value of all the MYU/Murex models has been increasing dramatically as more collectors show interest in them, and those that remain will continue to grow in value.
In 1973 a black striped version appeared, also a short type pen. Its model number is M-500BS. The letter "M" (for "MYU") plus the Pilot name was stamped on the cap. The black enamel stripes are recessed, away from exposure to rubbing. The finish was brushed steel. It retailed for 5,000 yen. This model is very difficult to find today.

In 1977 the integral nib steel pen was radically redesigned to be a full sized pen with textured section, flat nib and black stripe in a spring type clip that adjusted to any thickness of pocket fabric. This model was the first "Murex" pen. Perhaps this new name was meant to designate this bigger, improved MYU as the king (Rex) of the MYUs. It could also be a reference to the sea shell of the same name. Its model number is MR-500SS, and it is also displayed in the Pilot Pen Museum (Pen Station) in Tokyo since it represents a new line. On the cap the black letters "MR" (for "MYU-REX") plus the Pilot name are engraved. It retailed for 5,000 yen, much more than the price of the original MYU 701, and its design was technically superior to the original MYU, although not as radical in its looks. This model was designed for comfort and effortless writing.

Stainless steel Japanese fountain pens such as these MYUs and Murexes tend to come with tiny scratches. I have rarely seen one without scratches, especially on the barrel of pocket pens. This is because of the metal piece inserted into the cap which grips the barrel and keeps it firmly attached to the cap by friction. As soon as they put the cap on a new pen, it receives a tiny scratch, the first of many. This also explains scratches on the barrels of brand new plastic pens.

There is no need to be upset if you find scratches on your steel pen. If you look closely at the stainless steel surface you will see that it is brushed steel, which is basically a pattern of microscopic scratches covering the entire surface. Each scratch reflects light, so the more it's scratched, the more the pen sparkles. The longer you use your pen, the better it looks. You don't have to baby your MYU or Murex or any other stainless steel fountain pen.
http://www.stutler.cc/pens/murex/index.html