18G598 - Gearbox front cover centralizing tool.
Description from the Workshop Manual:
"In the case of front covers fitted with oil seals, the use of this tool is essential to assure concentricity of the front cover with the mainshaft, thereby obviating any possibility of oil leaks".

I think the book may be incorrect here. Early cars had no rubber seal, just the mechanical scroll seal in the front cover. Running clearance there is smaller than clearance around the mounting bolts, so a centering tool may be necessary to prevent mechanical contact in the scroll seal area. Later covers with the rubber seal don't care, because the rubber lip has enough flexibility to accommodate a lot of misalignment, so no special tool is necessary.