Rear spring threaded sleeves with 18TPI?

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I'm in the process of refreshing the rear suspension on my 1967 S3 SE. It was raced in period, and the owner at that time installed threaded sleeves and smaller diameter springs on the rear. He later reverted to stock rear springs. I'm planning on going back to smaller diameter springs, therefore I want to make use of the sleeves that are already there. However they appear to be threaded SAE, 18TPI to be exact. I have both SAE and metric thread gauges and the metrics simply do not line up. Meanwhile I cannot find 18TPI spring perches or locknuts. Everything is metric these days. I'm looking for information that may help me solve this dilemma.

Picture is not the best for this purpose but it (just barely) shows the threaded sleeve, with one threaded ring at the base. The original spring perches were trimmed for clearance by the prior owner.