Looks like they put it in to supress the reverse polarity surges caused by the collapsing field when the relay in the pulse module opens.
Any general purpose switching diode will do the job, say a 1N4004.

If you leave it out, expect the contacts on the relay to pit faster than normal, but that's the only consequence.

It is not shown in the 2005 SRT service manual diagrams, but a lot is left out of our service manuals.