Those are some very interesting custom 'muzzle brake' ports:

The receiver in your image is clearly marked as a 'Model GK' Mannlicher Schoenauer.

The GK, or gerader kolben actually designated a style of stock rather than any other difference from MC or MCA models of the same period.

Mannlicher Schoenauer commercial sporting rifles and carbines through 'Model 1952' all had the same stock profile (unless custom ordered) from the receiver rearward, though available in half stocked rifle or full stocked carbine (stutzen) form. Prior to WW2 they were also available as a handy 'Take Down Model:

'Prewar' MS stock dimensions

Take Down Model from 1939 Stoeger
Note custom options listed below image

Above is the standard stock, which would be replaced about 1956 with the raised cheekpiece shown below:

Not everyone liked the new high combed stock, so a 'toned down' version was also offered.
Steyr referred to the high comb as 'Monte Carlo' or Model MC as stamped on their receivers, and the medium rise comb as 'Monte Carlo Amerikanisch', or 'Monte Carlo All-Purpose' which were stamped Model MCA.

MC from 1962 Stoeger

MCA from '62 Stoeger

For those who preferred the old style stock, there was Model GK such as the receiver you've shown.

Model GK - Gerader Kolben

There was also a Model NO, which appears to indicate non - standard or custom orders.
Here is a previous thread regarding these models: http://forums.nitroexpress.com/showflat.php?Cat=0&Number=325014&Main=324972