HDD15 to 5BNC
13W3 to 5BNC 

13W3 to 15-Pin mini-D - Detachable Monitor Cable Connectors
(Cable - FRU P/N 96G2156 e.g. used for P70-6544) 

I got the 13W3 pinout from the port of my Image-I. It IS correct.
 

Signal Name	DB9	13W3	HDD15	DDC	DDC2A/B
Video, RED	1	A1	1	1	1
Video, GREEN	2	A2	2	2	2
Video, BLUE	3	A3	3	3	3
Video, return RED	-	-	6	6	6
Video, return GREEN	-	-	7	7	7
Video, return BLUE	-	-	8	8	8
HORIZ SYNC	-	5	13	13	13
VERT SYNC	-	9	14	14	14
Sync return, Horiz	-	10	-	-	-
Sync return, Vert	-	4	-	-	-
Sync return, COMMON	-	-	10	10	10
SELF-TEST	-	3	5	5	5
ID-BIT 0	-	6	11	-	-
ID-BIT 1	-	7	12	-	-
ID-BIT 2	-	1	4	-	-
ID-BIT 3	-	2	15	-	-
DDC - SDA (data)	-	-	-	12	-
DDC - SCL (clock)	-	-	-	15	-
DDC2A/B POWER	-	-	*	-	9
KEY (no pin)	-	-	9	*	-

Missing Pins

The key position  pin-9, above, corresponds with an obstruction  on many system unit video ports, this is intended to reduce the likelihood of something other than a monitor being plugged into the port and so offers limited protection to the electronics on the video adapter card. 
DDC uses this pin for power and reassigns the ID bits for DDC signalling. 

Older monitors do not use all the pin positions available. 
The following were manufactured with pins intentionally missing: 
 

Machine Type	Missing pins	Note
6312	9
6314	9 - 4,12,15
6319	9 - 12,15
6315, 21,22,24,25,27	9
6553, 6554, 6555	9	DDC supported
8503, 8504 (12M)	9 - 1,3,4,8,11,15
8506	9 - 1,3,4,8,11
8507	9 - 1,3,8,11,15
8508	9 - 1,3,4,8,11,12
8512, 8513	9 - 4,12,15
8514	9	all ID bits supported
8515, 8516, 9515	9 - 11,12,15
8518, 9518	9 - 4,12,15
9507	none	fully pinned
9521,22,24,25,27	9

Note that 6091I-19i and 9504 use the Image Plus 13W3  connector, which is self-keying by virtue of its three coaxial connectors, it is always fully pinned. 

FRU P/N 96G2688 = Standard ID bits, 13W3 -15-pin I/F cable - (supplied with the RISC System/6000) 
FRU P/N 96G2689 = Alternate ID bits, 13W3 -15-pin I/F cable 

Cable / Converter - FRU P/N Reference

FRU P/N	Description
40H2934	13W3 to 13W3 cable
52G3282 (52G3255)	15-pin to 13W3 cable converter
96G2156	13W3 to 15-pin cable providing DDC 'Plug and Play' capability.
96G2688	13W3 to 15-pin cable providing '1010' ID bits
96G2689	13W3 to 13W3 cable providing '1010' ID bits
96G2693	15-pin to 15-pin cable adapter providing '1010' ID bits
96G2694	15-pin IBM to 15-pin Apple cable adapter.

All shields are tied together to shield on HDD15 and pins 4,6,7,8,10,11

Pins 5, 12 and 15 are NC