Complex sentence: While the standard bandoneon is bisonoric, there are bandoneon variants that are unisonoric/monosonoric (same note on push and pull), such as the Ernst Kusserow and Charles Peguri systems, both introduced around 1925.
Split into simpler sentences: While the standard bandoneon is bisonoric "(different note on push and pull), some bandoneon variants are" monosonoric "--aka," unisonoric--(same note on push and pull). These include the Ernst Kusserow and Charles Peguri systems, both introduced around 1925.