1\4 or even 3/8" will fit or ask for the primary filter that fits a 1979 240D, its a $2 item that everybody carries. Then cut the hose when you find it and splice the filter into the system with hose clamps if nec. The once a year replacement large cannister filter is known as the secondary. Meanwhile immediately order a couple of secondary filters online. Gotta have decent fuel filters for the diesel, probly the most important single maintenance item. Primaries typically last 3-6 months, the secondary filter should be good for a year. Trust me, you dont want get lazy with changing the filters or your injectors and pump will go south plus the car will slow to a crawl.

(edit: Near as i recollect MB didnt produce the really cool glass bowl primary cannister in '65 - too many ham handed mechanics kept busting the glass, torquing the wingnut too tight.... '63 is probly the last year they had it. It dissapeared about the same time hollow camshafts with oil jets in the lobes vanished from production.