Looking to buy this 1969 Series IIa but it currently has an issue with shifting in to 3rd gear. No apparent catastrophic failure, just stopped going in to gear.

Owner said it may have something to due with a "stuck pin" in the shifter or worst case a synchro issue.

This vehicle may have sat for a long time and then it had this issue.

Any recommendations for approaching this problem without just dropping the tranny and taking it apart? If it was a synchro issue would it be able to shift into 4th gear? First and second gear still work.