Have your master key duplicated, preferably with an all metal key. I chose to use my stock master key.

If you choose to use your master key then you will just have to cut all of the rubber off. I cut the rubber off using a razor blade.

Open the switchblade key to the halfway position, look closely at the pivot point and you should see a small hole with a pin inside of it. Your goal is to get that pin out. Once the pin is out, the MKIV key should slide out.

Here is a picture of the key next to the extracted pin.

Next we will want to begin cutting the master key in order to fit it into the switchblade. Hopefully you have access to a Dremel® tool and bench grinder. These are not necessary but will greatly aid in the shaping of your key. Begin by cutting your master key away from the handle as shown below.

Once the stem of the key is separated from the handle you will want to begin shaping it to the profile of the MKIV key. Take your time and remove small amounts of material. You can always take away metal but you can never put it back.

Once your done grinding away enough material brush the key against the wire wheel to clean up any burrs. This will help when test fitting into the switchblade. Your product should look something like this.

Now you are ready to create the channel for the roll pin installation. Hold the switchblade in the half open position with the key inserted.  Have a friend carefully drill through the hole while the key is inserted. This will created a perfect channel for the roll pin to sit.

This is what the channel should look like when done drilling.

Clean out any shavings created by the drilling and re-insert the key. Install the roll pin back into the switchblade. Your installation should be complete!