Background: Recent years have seen a surge in cyber-attacks, which can be prevented or mitigated using software security activities. OWASP SAMM is a maturity model providing a versatile way for companies to assess their security posture and plan for improvements. Objective: We perform an initial SAMM assessment in collaboration with a company in the financial domain. Our objective is to assess a holistic inventory of the company security-related activities, focusing on how different roles perform the assessment and how they perceive the instrument used in the process. Methodology: We perform a case study to collect data using SAMM in a lightweight and novel manner through assessment using an online survey with 17 participants and a focus group with seven participants. Results: We show that different roles perceive maturity differently and that the two assessments deviate only for specific practices making the lightweight approach a viable and efficient solution in industrial practice. Our results indicate that the questions included in the SAMM assessment tool are answered easily and confidently across most roles. Discussion: Our results suggest that companies can productively use a lightweight SAMM assessment. We provide nine lessons learned for guiding industrial practitioners in the evaluation of their current security posture as well as for academics wanting to utilize SAMM as a research tool in industrial settings.