Originally Posted by Vintage_Cyclist

A commonly posted issue. There are multiple threads about this, with solutions ranging from hunting down the parts, modifying the keyhole shaped mounts and threading, and converting to brifter or bar-end shifters. I did the latter with my 85 Centurion Ironman.

Here's a list of related threads:

Modification: Old style Shimano downtube shifter mounts to later style - Bike Forums

Shimano 600 shift lever woes on a Centurion - Bike Forums

Do I have "B" Style Shifter Bosses? - Page 2 - Bike Forums

Shimano 4.5mm Type-A Shifters / Downtube Braze-ons - What are my options? - Bike Forums

Help Me Out With Shifter Boss Covers on This Miiyata 912 - Bike Forums

cable stops for unusual downtube brazeons - Bike Forums

Downtube shifter boss compatibility - Bike Forums

http://simplicityvintagecycles.com/2...aga-continues/

There's an online vendor called Loose Screws, which used to be bike oriented, but has shifted to firearms. They still have old bike stuff for sale though, including stuff that might work for you.

Shimano Dura Ace 4.5mm Downtube Shifter Screw w/D-ring - I used these to get cable stops on my shifter mounts, for use with bar end shifters. I removed the D-rings by putting the screws upside down in a vice and tapping them with a hammer to pop-off the D-rings. Having the screws is half the battle. These are them.

Shimano Z-401 Friction Downtube Shifter Set - can't vouch for these and the description incorrectly describes them as "Italian type", which they're not.

Shimano Dura Ace EX Downtube Friction Shifters Set - another bad description, but looks suspiciously like the correct shifters.