They are a mixture of hard training and site injections (deep tissue; probably eisclene).
Anybody who has very good calf genetics (Yates for example) will have long soleus muscles and thick gastrocs. Arnold has a thin, short soleus and all the mass is in the gastroc... that doesn't happen genetically.
When a natural (ie: no site injections) calf is flexed with the foot hooked (acute angle between foot and shin), then the inner head of the gastroc is a flat oval shape. When Arnold turned his leg to show off the inner head of his gastroc, as he did at the beginning of his 1980 Mr O routine, then the site injections are exposed... there is a lip of swollen tissue on the lower edge of the inner gastroc head.
Nasser el Sonbaty had the exact same thing... a lip of lumpy tissue at the lower edge of the inner gastroc.
It's a dead giveaway... just like a peaked lump in the middle of a stretched bicep in the upper arm.
If someone can dig up pictures of Arnold or Nasser doing this pose that would help illustrate what I mean.
The Luke