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The magnetic domain structure and properties of sintered Sm(CobalCu0.07Fe0.22Zr0.04)7.4 have been investigated, at different aging stages and applied fields. A low coercivity (0.2-2 kOe) developed during isothermal aging at 800°C for 16 h, while a maximum coercivity of 31 kOe was obtained after subsequent slow cooling down to 400°C (fully heat treatmented sample). Although the microstructural morphology remains unchanged for aging over 3 h, the observed magnetic domain structures are different. Flat or less wavy domain walls are seen in the magnets subjected only to the isothermal aging. The domain walls are wavy with a zigzag shape following the cell boundaries in the samples subjected to a full heat treatment. The evolution of domain structure with applied fields is consistent with the change of magnetization along the initial curve. This paper shows direct evidence of domain wall pinning in Sm2Co17-type magnets, where the degree of domain wall "waviness" is strongly related to their magnetic hardening.