Skindiver Magazine, 1978

At the end of the 1970s, a small company based in Norwalk, Connecticut began flogging a new line of quartz diving watches via mail orders in popular scuba diving magazines. "So what?" you say. "There were tons of quartz diving watches made in the '70s and '80s." And you're right, of course. Vintage mechanical diving watches have stood the test of time and reign king, while their quartz contemporaries--the red-headed stepchildren--linger in the shadows. But there are a few exceptional examples. This is one of them.

Part of what made the 23501 a stand-out in the watch community is the fact that it used the same Monnin case that was used on the Royal Navy issued dive watch made by Cabot Watch Company. The Ministry of Defence had replaced the Rolex Submariner with the CWC model, giving it immediate gravitas as a military-approved tool that is just as robust as the already legendary Sub. In turn, enlisted men in the US and elsewhere began abandoning Rolexs (which had alreadystarted its assent from tool watches into luxury time pieces) and Seikos--both of which were quite popular in the 1970s, particularly in Vietnam--and picking up rugged quartz watches like Chronosport's 23501 and UDT as they were inexpensive, reliable and tough. Hollywood caught wind of this affinity for the brand among troops and started strapping Chronosports on the wrists of characters like John Rambo, Angus MacGyver, and Thomas Magnum (more on that later).

The company

Chronosport Inc. was an American company based in Norwalk, Connecticut.

The case

The 23501 is based on the Monnin 844 case, which was used in a variety of dive watches, including the Heuer, Cabot Watch Company (CWC), Adura and Bessa. The case size is 42mm with 20mm lugs.

Founded in 1946, Georges Monnin (also known as G. Monnin and, later, Monnin SARL and Jean-Charles Monnin, Monnin Holdings SA.) was a manufacturer of watch parts and cases for high end brands, based in Besançon, France not far from the border with Switzerland. The case used on the Chronosport Sea Quartz 30 has been credited with saving the company Heuer as it formed the basis of its first dive watch. Much has been written about the Heuer 844 (launched in 1979), but the real star here was the perfectly designed case that was picked up by many other watch manufacturers and fitted with a wide variety of automatic and quartz movements.

The Monnin 844 case is registered as Swiss patent number 503.305, which is why you will find the following engraved on the inside of the case back:

BREVET + 503305

M.R.P. SA

SWITZERLAND

M.R.P. SA is a watch case manufacturer that is still in business today. https://www.mrp-sa.ch/

It is unclear how or when these two companies are related.

The dial

Mark I dial (No-numbers)

The Chronosport 23501 began with this dial, likely in 1978 or 1979. The Mark I variant of the 23501 has no 6, 9, 12 numerals. The dial also has QUARTZ written on the dial above the six o'clock marker. SWISS MADE is written across the bottom of the dial. These examples have serial numbers engraved below "30 ATM" and "23501" on the case back along with the script "ALL STAINLESS STEEL SWISS MADE WATERRESISTANT SHOCKRESISTANT" in a circle.

This was the screen worn version worn by Tom Selleck in the Magnum PI pilot episode "Don't Eat the Snow in Hawaii," which aired in December 1980. Magnum then wore the Mk II version in subsequent episodes before it was replaced with the Rolex GMT Master in season 4 (1983).

Here is an example that was purchased in October 1979 (serial 3-20495).

Here is another example (serial 3-20640)

https://www.shpock.com/en-gb/i/XjCRWTC7zxZWF3V9/vintage-chronosport-magnum-pi-watch-breitling

Mark II dial ("Quartz" italic script)

The Mk II dial made a decisive departure from its predecessor by adding large 6, 9 and 12 markers. This version is differentiated from the later versions by the the italic scripted "Quartz" on the dial above the six. It also has "T SWISS MADE T" split on either side of the six o'clock marker, indicating that tritium has been applied to the dial for luminescence. The tritium marking is not present on later dials. This exact dial and case can also be found bearing other brand's logos (including Rutina and Tegra), but still often bears the Chronosport logo on the case back.

This Mark I dial is the exact screen worn example used in the first three seasons of Magnum P.I. (in the first episode of season four, Home From the Sea, there is a watch-focussed episode where Thomas is famously bequeathed a Rolex GMT Master by his pilot father). The Chronosport does appear in a few other episodes in that season that were presumably filmed before Home From the Sea.

Interestingly, the case back for the Mark I variant does not have a serial number engraved below the numerals "23501."

Here is an excellent example with original box and booklet.