Another little step forward for the project, a DM74S571AN arrived from Buzby today, so... (image).
To be honest I'm posting this partly to cover my own rear exit, because Buzby's original SOC-supplied IC2 (marked '1o', offset to one side upper right of the picture) died not ten running hours after it was originally working here. So here is proof that the original IC3 and the replacement IC2 were working on this day. I couldn't find a newspaper headline to include in the shot.
I ran these for about an hour and a half in my original MK14, because that machine has the regulator mounted offboard on a substantial heatsink whereas my issue VI replica only sports a skimpy little black number.
One thing I did notice is that the original IC3 (Non-A version, large label on top) runs hotter than the replacement IC2 ('A' version, no label) so I would suggest using small labels like the sticky 'dots' SOC originally used on these new-OS PROMs and placing them at one end of the IC rather than over the middle, which is where SOC originally put them.
We can now add the DM74S571
AN to the list of specific devices known to have been tested in the MK14.