The Old School Audio Track Pack is an 8 space 500 series lunchbox type rack with hardwired power umbilical and external PSU. It differs from both the VPR spec and the newer 51X spec. OSA ceased production, and info is scant on the ground. Here's what I found.
pin
1 not connected - to anything, anywhere
2 + out, hardwired to XLR pin 2
3 -2 out, jumper to XLR pin 3
4 +4 out, jumper to XLR pin 3
5 M and F XLR pin 1's / not connected to card edge pin 13
6 one jumper for link to adjacent, and second jumper for bus connection to all 8 channels; bus has no external connection
7 not connected
8 hardwire XLR pin 3
9 not connected
10 hardwire XLR pin 2
11 link jumper to adjacent
12 +16VDC, power pin 3
13 PSU common, power pin 2 / not connected to card edge pin 5
14 -16VDC, power pin 1
15 +48VDC, power pin 4
output XLR's shell connected to chassis
circuit board has points for additional parallel input and output XLR's
VPR standard for comparison
pin
1 chassis
2 +out +4
3 +out -2
4 -out
5 common
6 525 link
7 -in -2
8 -in +4
9 +in -2
10 +in +4
11 gain trim
12 +16VDC
13 PSU common
14 -16VDC
15 +48VDC
pin
1 not connected - to anything, anywhere
2 + out, hardwired to XLR pin 2
3 -2 out, jumper to XLR pin 3
4 +4 out, jumper to XLR pin 3
5 M and F XLR pin 1's / not connected to card edge pin 13
6 one jumper for link to adjacent, and second jumper for bus connection to all 8 channels; bus has no external connection
7 not connected
8 hardwire XLR pin 3
9 not connected
10 hardwire XLR pin 2
11 link jumper to adjacent
12 +16VDC, power pin 3
13 PSU common, power pin 2 / not connected to card edge pin 5
14 -16VDC, power pin 1
15 +48VDC, power pin 4
output XLR's shell connected to chassis
circuit board has points for additional parallel input and output XLR's
VPR standard for comparison
pin
1 chassis
2 +out +4
3 +out -2
4 -out
5 common
6 525 link
7 -in -2
8 -in +4
9 +in -2
10 +in +4
11 gain trim
12 +16VDC
13 PSU common
14 -16VDC
15 +48VDC