The Laso company (note bought some from Laso) makes aftermarket pre-chamber retaining rings with a slot for the 617.952 and earlier engines but I don't know if they are the same size as yours. The Injectors screw into the retainer ring.

I thought that Pelican had someone that would look up and source out parts.

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84 300D, 82 Volvo 244Gl Diesel