This is a Prototype Board for the Expansion-Port of the Commodere C64. Its goal is to ease cartridge development and experimentations.
It's for sure not the only C64 prototype board out there but it comes with a fairly unique design.
Last Update: 24.07.2021 
C64 Expansion Port Prototyp Board
The features of that board are:
The Expansion Port is passed through to a cartridge slot at the end of the board.
Every single signal line to the additional Expansion Port can be cut by a jumper. Via a solder blob the line can be rejoined again.
A Reset-Button is provided by design.
The overly long board can be shortened by carving along the jumpers into both sides of the board with a knife and break it through afterwards. The board is perforated along the jumpers.
The main board fits into a standard cartridge shell.
With spacer the secondary board can be mounted on top of the main board. The signals are passed through via pin header.
The stacked board is still fitting into the cartridge port of the C64.
The schematics of the prototype board.
And finally the layout of the prototype board.
The current version can be bought soon at

(I'm not associated with in any form. I've only ceded the design of the board for use to him.)
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