The chip markings on the old chip refer to the individual modules. They're used by Micron, Corsair, Ventura etc on a variety of 128Mb and 256Mb chips. See
here for an example. I can't pin down the part number to identify the chip manufacturer.
The specification for the Hynix is:
Module Size: 128MB
* Package: 168-pin DIMM
* Feature: SDRAM, PC133/100
* Configuration: 16Meg x 64 (16x4 Config)
* DIMM Type: Unbuffered
* Error Checking: Non-parity
* Speed: 7.5ns
* Voltage: 3.3V
* SDRAM Timings: CL=3
The equivalent spec for the micron chip above is as below:
Module Size: 128MB
* Package: 168-pin DIMM
* Feature: SDRAM, PC133/100
* Configuration: 16Meg x 64 (16x4 Config)
* DIMM Type: Unbuffered
* Error Checking: Non-parity
* Speed: 4.5ns
* Voltage: 3.3V
* SDRAM Timings: CL=2
There's no obvious mismatch based on the chip markings, so I wouldn't throw blame on the assistant in the shop. You could ask them to swop it for another chip from a different manufacturer but, as suggested above, you'd be better off exchanging it for a single new 512Mb chip to replace both. That way there won't be any possibility of a mismatch. They're cheap enough.