I have a couple of thoughts on this watch.
1st - It likely is platinum. Lots of old jewelry, as it was pointed out above is often unmarked regarding the metal content. However, this mostly applies to gold items. I have several pieces of vintage jewelry that range between the 1830s and 1910 and most of the gold items (9k to 14k) are unmarked. These are likely one-of items that were made by a local jeweler and it was custom to not mark them with anything other than a makers mark. Higher end stuff and items that were mass-produced (think watch cases) however are generally marked. Platinum is different: it is almost always marked. Platinum wasn't generally used in jewelry until the very last part of the 19th Century; rarely is it unmarked. I have a platinum watch (ca. 1920) a family piece and it came to my possession because no one thought to look inside the case (where is is marked Plat/Irid). Yours will likely be marked on the inside of the case.
2nd - Agassizi is another possibility: google returns a number of hits that suggest there is a relationship between the two companies in the beginning of the 20th century. Also, the Swartchild's catalog shows several Agassi movements with similar bridge styles.
3rd - The case bezel is likely threaded rather than being a snap type. If you want to try it, use one of the rubber ball case openers and give it a light twist on the back. If it turns and loosens than you have confirmed this. If it remains tight or wont turn at all it then may be a snap back. You might then consider taking it to a good watch maker.
*until white gold or platinum was used in jewelry, sterling was the only white metal choice to show off diamonds (think late 1700s to mid-1800s). I've known of one jeweler that bought a $5 'costume jewelry' ring at an estate sale that was actually several mine cut diamonds set in sterling.