Thanks. If you look in ioreg for PCI0>AppleACPIPCI>IGPU>IntelAccelerator>IOCFPluginTypes , there will be IOAccelerator2D.plugin.
While the original imacs with hd2500 have IGPU>Gen7Accelerator.
Btw. these original imac use
"AAPL,ig-platform-id" = 0x06, 0x00, 0x62, 0x01 or 0x07, 0x00, 0x62, 0x01...