Seiko Prospex SRP637 and SRP639 aka Monster Tuna

*I am back from travelling. Time to do the brief review as promised.


The temptation is great, had to buy the Seiko Prospex SRP637 and SRP639 – nicked named Monster Tuna.

The 3 new Seiko Prospex model available are SRP637, 639 and 641. They are now upgraded to the Prospex status ! with the NEW “X” logo printed on the dial. They are suppose to replace the earlier plastic shroud model aka Baby Tuna SRP233K, SRP234K and SRP236K.
The new model has been nick named the Monster Tuna (similar dial and hands to the Seiko Monster) and Sardine here locally. Obvious changes between model are the shroud (plastic vs steel) and the hands.

Photo 8-12-14 10 04 36 am

I decided to purchase only the SRP637 and SRP639, the SRP641 is available in Black PVD only ; which is not my taste.

Seiko SRP637 (Bracelet)
Lets start with the SRP637. This is the bracelet model.

Diameter: 47.5mm (excluding crown)
Lug width: 22mm
Lug to lug: 50mm
Thickness: 14mm
Water resistance: 200m
Movement: 4r36 Movement (automatic/ hand windable/ hackable)
Shroud: Metal
Crystal: hardlex crystal
Bracelet: Stainless steel metal

The SRP637 wears big on the wrist. The metal shroud is thick and is also a welcome upgrade. Comparing to the earlier plastic version, the metal shroud will definitely outlast it. The bezel turns firm and accurate. The hands is similar to the monster with Seiko Lumbrite. The 4R movement is as usual accurate and reliable. The crown is threaded and has 4 turns.  The chapter ring is made of plastic and needs careful selection when buying your watch ; as i have spotted some with mis-align ones !

Photo 13-1-15 3 15 47 pm  Photo 13-1-15 3 16 15 pm Photo 13-1-15 3 15 59 pm

The bracelet looks similar to the Seiko Monster except that its 22mm.

Photo 13-1-15 3 13 17 pm

There is a build-in scuba extension on the bracelet.

Photo 13-1-15 3 13 51 pm

The springbar used is the Seiko Fat springbar.

Photo 13-1-15 3 12 44 pm

The compulsory lume shot, as usual BRIGHT !

Photo 13-1-15 3 18 42 pm

Here is a shot against the earlier version (mod with a steel shroud).
On the left, the earlier version.
On the right the Prospex SRP637

Photo 21-12-14 11 32 49 am

Seiko SRP639 (Rubber strap)
This model Prospex SRP639 is selling like hot cakes here and is currently out of stock in many places. I believed its due to its slight cheaper price and the colour coordination.

Photo 13-1-15 3 46 21 pm
Similar to the SRP637, except the bezel is in black. The markers is yellow painted from 12pm to 3pm. The minutes hand is also yellow in colour. The second hands is painted yellow at the bottom. The combination is very pleasing. I can see why its popular !

Photo 13-1-15 3 46 31 pm

Here is a wrist shot.

Photo 13-1-15 3 50 47 pm
For the rubber strap, its using the famous Z22 series rubber strap. This Z22 Seiko rubber strap needs an update ; its a little hard and will needs some boiling trick to make it comfortable for wearing.

Photo 13-1-15 3 51 05 pm

*Seiko has made the SUN019,21 and 23 model with nice and soft rubber/silicon strap. For a Prospex watch…….. an update on the strap is needed.

Seiko SRP641 (Rubber strap)
This is the all black PVD model. I did not buy this model as PVD is not my taste (hard to swallow when there is a nick).
*Photos below are not mine but source from the net.

I can see the difference are : gold hands, gold crown, gold shroud securing screw and the dial are painted with gold markers.
Photo 10-12-14 4 15 53 pm

Photo 10-12-14 4 15 59 pm

Photo 10-12-14 4 15 56 pm

Photo 8-12-14 10 06 23 am


OK, here are some photos with some of the Tuna that i have. Enjoy !

Photo 13-1-15 4 04 34 pm



  1. […] the temptation, bought the Seiko Prospex SRP637 and SRP639. Had done a brief review below. Enjoy ! Seiko Prospex SRP637 and SRP639 aka Monster Tuna | seikoparts Like me ! Like me ! Visit my BLOG. Reply With […]

  2. Hello – I enjoyed reading your article, as I have just purchased the SRP639. I have a question: Can I install the silver bezel on the 637 to replace the black & yellow bezel on my 639. And if I can, would you perhaps know where I may acquire the silver bezel? Thank you for your help. My Best, Paul Hill

    1. Suggest you scout the Seiko forum, or purchase thru the service centre.
      Another way is to sell the SRP639 and buy the 637.

  3. Besnik Nadzaku · · Reply

    love your article,thanks for the time to write it wa wondering if you know if all three watches the shroud interchangeable.thanks gor reply.

    1. Thanks for your compliment.
      Yes, all 3 watches shroud are interchangeable.

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