Complex sentence: Until 1983, when the Dr Martins brand changed ownership, they shared the work of producing Dr. Martens boots and for a short time until the contract ended produced boots labelled "Dr Martens Tredair".
Split into simpler sentences: Until 1983, when the Dr Martins brand changed ownership, they shared the work of producing Dr. Martens boots. For a short time from then until the contract expired they produced boots labelled "Dr Martens Tredair".