hello! i have been having this same problem recently with my 5 speed in my 1975 620. i did a 4 to 5 speed swap and since then have worked on the tranny a dealing with this problem as well as having fixed a sheared roll pin. in the case of the sheared roll pin, it broke on a selector and this results in the feeling of a free swimming shifter where you cant find any of your gears, its as if they have all disappeared. i only bring this point up because a broken pin is a totally different feeling than this problem ohmrchristopher is dealing with. recently my 5 speed got stuck in first as i had the same symptoms of being able to shift to 5 and R and swing straight thru neutral until i hit 1/2 in which the gate finger was blocking me from selecting 1 or 2. i had hoped it was a sheared roll pin again that had somehow lodged the selector in 1st gear and was stuck, but upon cracking the transmission open (removing tailhousing) i was easily able to move the 1/2 gate back to neutral, and no sheared roll pins! it seemed to check out on visual inspection and the fact i was able to shift thru the gears using my hands to select gears with the gates gave me reason to believe it was a simple fluke problem. this instilled in me the idea that there were no broken roll pins or mechanical issues to address anymore. i reinstalled the transmission and the gears shifted flawlessly for a few days driving 25-40 miles a day. left for the weekend and this morning went to go to reverse out of the driveway and low and behold my tricky little tranny had crept back into first while i wasnt looking! same symptoms, cant get it into the 1/2 gate, stuck in 1st when the clutch is let out. this all being said i am left to guess that this could be a check ball problem? which i doubt because the check balls are operating correctly by the fact i cannot shift into 3/4 right now because 1/2 is engaged. so maybe it is a selector rod problem then? the selector has crept out of its original position and when operating correctly is floating closely between 1/2 and 3/4 so its able to engage 1/2 and then slip out into the rest of the gates? or a worn selector? which i also doubt because upon visual inspection it seems to be fine. im sorry i dont have an answer but i figure all the info i have gathered, although not decisive, will benefit this group problem solving going on.
random notes i have acrewed in my journey
-this happens to me only after the car has been parked, i drive into my driveway in 1st and engage the clutch and move the shifter into neutral to park the vehicle
-before this happens (in the days prior to getting stuck) i feel a stiction as i shift from 1st to 2nd, it doesnt stop the shift from happening, but it feels as though there is resistance to the shift
hope any of this helps.