Does anyone know what are those Lehmann (or Lehman) Brothers and why they aren't mentioned in other sources?
They are mentioned plenty of places, but not usually in connection with automobiles. "Lehman Brothers" was originally a dry-goods proprieter founded by Bavarian immigrant Henry Lehman in 1844. in 1855, when cotton was king, Henry and his brothers began to accept cotton (and other commodities) as payment for the goods they sold. A second line of business, focused on cotton trading, was begun at that time. When Henry passed away, the remaining Lehman brothers reshaped the company to a commodities trading and brokerage firm.
The firm weathered the depression by being shrewd investors in carefully selected start-up companies. We call these kinds of investment firms "investment capitalists" today. (Except, of course, as businesses in the US wait for the Obama socialists to drop the other shoe, venture capital investments have largely dried up in the last few of years).
Lehman Brothers was owned by American Express for 10 years in the 1980's and 1990's. AMEX divested themselves of their investment service lines in 1993-4.
After a period of financial shenanigans, and a deep dependency on the sub-prime mortgage business, Lehman filed for bankruptcy in 2008. This remains the single largest bankruptcy in the history of the US.
Lehman Brothers' connection with this car is a mystery to me.