TCM FG2N4T 2780636 production date is between 07/1/1973 and 09/1/1973. The stamping of M1026 is a Clark Equipment format and indeed a mast that they produced. I am unable to find any indication that a Clark mast was installed on this truck however Clark Equipment and TCM were in joint ventures for many years after WWII. If your mast is three stage and has one single free lift and one single rear lift cylinder it may be a TCM FHM277 and the seal kit for the single rear cylinder would be TCM part 23460-89811. However I can make no guarantee of this to be accurate given the age and limited recourses available to me. Your best choice would to have a local hydraulic supplier or repair shop match the seals or even rebuild/repair the cylinder. If I may be of further assistance please contact me, the email is in my signature line.
Regards,
Doc