'51 Frazer three-on-the-column w/o warning stopped engaging in reverse. Gearshift does not engage but I cannot find anything broken or binding. No cotter keys missing, nothing loose I can discern. I believe it is the frontmost linkage that should be UP when in reverse, but even when I'm under the car pushing it up, it will not remain up. There is (and has been) a lot of slop but it's shifted before now. I am able to slide the two linakges back and forth a good bit under the car but not sure what impact this may or may not have. 1st thru 3rd appear alright, it's just reverse I think. What should I look for?