A boutique investment group has quietly pocketed a slender, 200,000-foot glass tower overlooking Bryant Park for more than $120 million — and intends to grow it by 70,000 square feet for residential apartments.

The Springs Mills Building at 104 W. 40th St. — just west across Sixth Ave. from the Bryant Park Hotel — was sold by Aby Rosen and Michael Fuchs of RFR Realty without brokers.

The exact price could not be learned, but similar properties are selling for around $500 a foot.

Buyers Mermel & McLain Management partnered with Washington-based pension fund ASB Capital.

M&M's M. Myers Mermel and Caroline McLain will manage and lease the building and also redevelop it.

Sources say they have already hired Skidmore Ownings Merrill to plan a major renovation of the elevators and lobbies as well as an expansion upwards of the 20-story building, which has magnificent views of Bryant Park.

Neither Rosen nor Mermel returned calls for comment.

City records show RFR bought the building in 1999 for $29.5 million.

Along with various partners, Mermel & McLain also own the boutique office building at 660 Madison Ave. where rents run more than $100 a foot, as well as 48 Wall St., which is co-owned with Kent Swig.