Further information on calligraphy piece? (Characters identified: 斿于⿰坴丮)
The characters are 「斿于」, or using ideographic description sequences: 「斿于⿰坴丮」. These are stand-ins for what we would now write as 「游于藝」; 「斿」 is the right hand side of 「游」, and 「」 is a seal script form of 「埶」, forming the middle part of 「藝」.
The phrase referred to is a line from The Analects: Transmission of Learning:

《論語・述而》
子曰：「志於道，據於德，依於仁，游於藝。」
The Master said, &quot;Let the will be set on the path of duty. Let every attainment in what is good be firmly grasped. Let perfect virtue be accorded with. Let relaxation and enjoyment be found in the polite arts.&quot;
Translation by James Legge. See Chinese Text Project.