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Some male IDC and Dupont connectors have a pin missing, with the corresponding female connector having that hole blocked up with some plastic, so that the cable can only be mated one way.

I can however only see for sale female IDC and Dupont connectors that have all their holes unblocked.

I am having no luck figuring out how you are supposed to block off the keying holes yourself. Is there a particular plastic piece you insert to achieve this, or is it just a matter of cramming the hole with whatever you can find and perhaps melting it in place?

The professional ones look either moulded or as if a plastic pin has been pushed into the hole which would be a nice touch, but I have no idea what this plastic pin may be called and all my guesses (key, block, filler, etc.) are coming up blank.

Is it possible to buy these keying pieces for both IDC and Dupont connectors, and if so what are they called?

Here is an example pic from Google showing a keying pin on a Dupont connector:

enter image description here

And another showing a blue keying pin on an IDC connector:

Key example on a 40-pin IDC connector

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    \$\begingroup\$ All I have worked so far with are moulded out of one piece. \$\endgroup\$
    – PlasmaHH
    Dec 23, 2016 at 12:46
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    \$\begingroup\$ I would call it a polarizing pin. \$\endgroup\$
    – Steve G
    Dec 23, 2016 at 12:54
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    \$\begingroup\$ related: electronics.stackexchange.com/questions/262850/… \$\endgroup\$
    – dim
    Dec 23, 2016 at 13:58

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It may depend on the manufacturer/vendor, but Digikey seems to sort them as "Keying Plugs" or "Keying Pins". See below for an image of what they look like, this one is for a TE Connectivity plug.

enter image description here

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"Molex Polarizing Key for use with 2.54mm Pitch C-Grid and SL Connectors" will help you find them for sale

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