Associations and their certificates.
Ludwig Freytag can do many things.And, obviously, may do so as well.

Gemeinschaft für Überwachung im Bauwesen e. V.
Society for Monitoring in Construction
Certificate of competence in protecting, repairing, joining and strengthening
https://www.gueb-online.de/
Deutscher Verein des Gas- und Wasserfaches e.V.
German Gas and Water Trade Association
DVGW certificate G1+pe, W1+pe, R2;
AGFW certificate FW1: cu, ku, s

Deutscher Verband für Schweißen und verwandte Verfahren e.V.
German Association for Welding and Related Procedures
Comprehensive qualification certificate as DIN 18800, part 7
https://www.die-verbindungs-spezialisten.de/
Technischer Überwachungs-Verein Norddeutschland e.V.
Technical Monitoring Association of Northern Germany
Certification according to ISO 3834-2;
Approval for specialised company according to Section 19 German Water Resources Act (WHG);
Approval for specialist company as per TRB 200/AD-Merkblatt HP O

TÜV Rheinland
Quality management as per ISO 9001:2015;
Safety, health and environmental protection system according to SCC regulations
Environmental management as per ISO 14001:2015

Deutsches Institut für Bautechnik, Berlin
Approval for shear-resistant adhesive joints between steel plates and reinforced concrete components or pre-stressed concrete components
https://www.dibt.de/
Bundesamt für Wirtschaft und Ausfuhrkontrolle
German Federal Office of Economics and Export Control
Approval for NATO infrastructure tenders
https://www.bafa.de/