The VSU-2 is a drop in replacement for the Stern VSU-100 speech board. It contains additional features like volume control, LED indication, and UART programming. All 6 original games are supported by default, plus experimental support for Gamatron (pinstar version, sonic is untested), as well as extra memory space for custom speech ROMs.
Mounting holes are identically spaced to the VSU-100, and original connectors fit these pin headers, making a swap quick and painless.