Key missing. Aluminum. Period time repaired with iron parts to make it fully working again. That definitely happened during its active use. Professionally reinforced with iron to the bottom part of one link as well (aluminum part shows cracks). The side with the repair is hold holding the links anymore. The inside part closing them seems to be missing.
"17404" numbered. DEUTSCHE POLIZEI D.R. PAT. AUGUST SCHWARZ BERLIN S.W.11 marked
