Congratulations on your DTV and Thank You for your report, but the Thai Embassy in Seoul's website is not particularly precise about what documents are needed for those who are not residents in South Korea and the Thai Embassy in Seoul's verbage seemed to imply that only residents of South Korea can apply there....
But, if one looks at the details for a Thai Tourist E–Visa (no details for DTV E–Visa), then it seems, from your experience, an Entry Stamp/sticker upon arrival into South Korea is acceptable for DTV applications of those not residents in South Korea....
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