It's from outside the EU, and the specifics weren't announced. Probably someone that operates NATO-caliber artillery, which doesn't narrow it down much.

I'd guess that it might be from Turkey. Apparently Turkey was willing to sell some rounds, but Greece was opposed because they didn't want money going to Turkish military contractors. Could be that Greece was willing to drop objections.

This is from a week ago:

https://greekcitytimes.com/2024/02/23/greek-veto-blocks-european-financing-for-turkish-drones-and-artillery-for-ukraine/

Reports indicate that the supply of Turkish-made Bayraktar drones and artillery shells to Ukraine, which were due to be financed with European funds, has been vetoed by Greece, Cyprus, and France. These three countries reportedly coordinated their efforts to restrict the transfer of military assets to Ukraine.

EDIT:

Turkey, South Korea, and South Africa were mentioned in this article as possibilities a month ago:

https://www.msn.com/en-us/news/world/czech-republic-ready-to-supply-ukraine-with-artillery-ammunition-pending-partner-funding/ar-BB1irsFX

According to a Feb. 1 article by Politico, Czechia has taken the lead in proposing a collective funding strategy to acquire 450,000 shells from outside the EU, possibly from manufacturers in South Korea, Turkey, or South Africa.

I don't know if Czechia actually mentioned them, or if that's just the author guessing as to viable sources.

If it's South Korea, it's going to have to be a ring trade -- like, EU member sends some of their emergency reserves, South Korea backfills the hole. That is, as I understand it, what the US did -- South Korea has constitutional restrictions that would prevent providing aid to Ukraine, but has been okay with doing a ring trade, which has a similar effect.

155mm is a NATO standard, NK uses Soviet 152mm. But it's quite possibly from South Korea, at least a part of it.