For the most part the MGs in the standard positions were mounted using an Ikaria ball mount.
Attachment 11776 Attachment 11781Attachment 11777 Attachment 11778 Attachment 11779 Attachment 11780
Source: As indicated, Vladimir Kuligin (via eBay), or various handbuchs.
If any other positions were field modified to carry extra guns then these were generally more makeshift swivel/bracket type devices. As you can see the MGs were mounted in many different ways.