Nice adapter you have there.
Heat it up with a heater gun and the 123 should slide in with no effort. Make sure you do not have the o-rings fitted. These adapters were made a piece and i know that some were slightly narrower than others.
When the adapter is hot just slide and do not use any hammers etc. If it does not fit you can try again by having the 123 housing very cool/freezing. In one case i had so sand slightly the 123 but do not sand the adapter. The size of fitting and not fitting is a few hundreds of mm so be carefull with sanding. Once the adapter is on the 123 it is very hard to get it off again. Make sure it does fit. Trying to heat to take it off again does also heat the 123 housing.
Try fit the 123 first with the drive gear attached and have it at the tdc. Make sure the rotor does point to the correct contact in the cap and the led of the 123 is just on. It is in the 123 manual how to set the static timing. Then mark on the 123 housing the two fixing holes of the engineblock. Then you know how to put the adapter and where the slots must be.
When is is not correct it can be that you can never adjust the 123 at tdc because the orientation of the distributor is wrong with the slots. I did made two studs in the engine because it is impossible to have bolts through the adapter once fitted.
Good luck.
Jeroen
When all goes ok you should end up like this:
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