FLASH FORWARD TO 2018
I was revisiting the Transpo regulator issue for a new job I undertook on my '99 300D which finally needed a new Alternator. I discovered Transpo was bought out in 2004:
"Transpo Electronics was founded in 1978 by Frank Oropeza, a former Martin Marietta electrical engineer, as a home-based business. Mr. Oropeza saw a need in the automotive aftermarket for quality domestic products that rivaled overseas manufacturing pricing. In 1981, Transpo also realized that technological changes in the industry would require smaller, more advanced packaging concepts, and invested into thick film technology. This foresight gave us a head start on the competition and helped develop the aggressive style that we still practice today. In 1987, we moved into a new 57,000 square foot manufacturing facility. In 1992, we expanded our facilities to a total of 77,000 square feet and in 1997, Transpo completed the expansion of our World Headquarters in Orlando, Florida, increasing our state of the art facility to over 125,000 square feet.

In March of 2004, Transpo was acquired by Wetherill Associates Inc. (WAI) of Royersford, Pennsylvania. WAI is the North American leader in the distribution of automotive electric aftermarket components. The merging of the two companies created the perfect marriage of product offerings and business disciplines that allows us to offer our customer the best products at the best prices with fast, economic delivery.

Transpo manufactures quality replacement products for the professional Electrical Rebuilder for most make and models of vehicles, including components for automotive, truck, marine, industrial, heavy duty, small engine, bus, RV and specialty markets. With over 2,000 available products, Transpo is recognized worldwide as the industry leader in voltage regulators and rectifiers. Our products can be found in alternators, distributors and engine environments worldwide. Our pledge is to supply the finest quality products available and is dedicated to exceed our customer's expectation."
END of ANNOUNCEMENT

I can't find a Transpo equivalent supplier at the moment but I can assure you Transpo's German website is dead.

WAI is using a website to list equivalents to Bosch and other brand alternator regulators however so maybe they are still a valid supplier, I'll et someone else decide. Here is a link to that cross-reference they host:
https://store.alternatorparts.com/bosch-2.aspx
DDH

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!