I’m going to close comments:
1. The STV and the extension of stay was a badly planned, badly thought out visa. MOST immigration offices do not know what to do with an STV because so few people entered on this visa. A safe assumption would be that the OP can not stay past September 30th. There are (non-credible) reports of people being stamped in longer, past October.
2.The STV can not be converted to another visa, does not allow employment, and no visa agents would even touch the STV.
3. An extension of stay can be acquired at MOST immigration offices when there is 30 days remaining, but some allow extension of stays to be acquired when there is 45 days remaining. I have seen no creditable reports yet of how most immigration offices are treating the STV and the 90 day extension of stay based on STV.
4. The last day a person can enter on an STV and qualify for the extensions is July 3rd. If a person will enter Thailand after that date, no extensions can be acquired.
Very few embassies still have their STV page up. Here is one from The Hague:
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Thank you