Eugene,
Its called an MMJ cable, Modified Modular Jack. The price from the Stonewall
link in Bills mail seems typical commercial.
Pretty some one in the USA was making these for less.
I have the bits to make some in the UK but shipping gets expensive.
Dave
  -----Original Message-----
 From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of W2HX via cctalk
 Sent: 15 June 2018 03:46
 To: General Discussion: On-Topic and Off-Topic Posts
 <cctalk at classiccmp.org>
 Subject: DEC VT 420 comm cable?
 Hi friends,
 I just received a refurb'd VT-420 with green screen. I see that I need a
special
  6 pin/modular-type cable for the comm port. I located
the adapter to get 
to
  DB25 ( 
https://www.ebay.com/itm/262041938994 ) but I
need the
 interconnecting cable. Anyone know what it is called? Anyone have one for
 sale?
 73 Eugene W2HX