1/4" Hex shank TCT hole saws are sold in Amazon Japan from size 12mm to 56mm with 1mm increasement

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Amazon.co.jp: Carbide Hole Saw, Stainless Steel (Hexagonal Shaft, 0.25 inches (6.35 mm)), T.C.T Material, Drilling Bit, Electric Drill, Hole Cutter for Metals, Includes Springs, Durable, Hard Alloy, DIY Case (φ53)

[Altan]

Here is another brand called "CSOOM" which has better reviews.

 

Amazon.co.jp: CSOOM Hex Shank Alloy Open Carbide Hole Saw Hole Cutter Circle Cutter Cutting Drill Bit for Woodworking Metal Stainless Steel Aluminum DIY Perforation Plumbing Hole Saw Water Pipe (1.4 inches (35 mm) : DIY, Tools & Garden

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Here are some photos 

14mm

 

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22mm

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30mm