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G. Skill 32GB DDR3-2400 at Amazon.com
12GB (1 x 4GB + 1 x 8GB) PC14900 DDR3 1866MHz SO-DIMMs 204-Pin Memory RAM Upgrade Compatible with 2015 (Late) iMac 27" w/Retina 5K Models and PCs (OWC1867DDR3S12S) at Amazon.com