I found an indie who will replace the front seal on my compressor if I bring him the correct seal. I don't know if the front shaft on the 300SD's compressor is 14mm or 17mm.

This is what I found on Ebay for 14mm shaft: AC GM Compressor Lip Shaft Seal R4 V5 HD6 HR6 HR6HE DA6 HT6 A C Double Lip 14mm | eBay

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'81 300SD

If there is an original GM part number on the compressor you might be able to research what seal it has. Past that going by make / model / year because the compressor may have been changed.

The only solution would be to pull the clutch plate and measure the shaft at the seal. Measuring at the clutch drive might not be accurate.