Unfortunately I do not have any information on whether this document is a pure censorship or an Israeli foreign exchange control. The Hebrew closure strip is more indicative of foreign exchange control, as is the backed rectangular stamp with handwritten note 483/9, which could also confirm the registered letter. I could not find any information at Norman Gladstone's 'Postal Censorship in Israel 1948-1978'. Does anyone have an explanation?
First Day Cover from 1.6.1951 transported via Zurich/Geneva (SR, TWA, El-Al) or Rome (KLM) to Tel Aviv and further to Holon (also called Cholon), with auxiliary registration label St. Gallen UPU (used up from 09/1938) and special cancellation p. 325, PÉN (duration of use 21.5.-2.6.1951).
Swiss postal abroad up to 20g = 40 Rp. (1.3.1948-31.3.1959) plus registration fee abroad = 40 Rp. (1.3.1948-31.3.1959) plus airmail surcharge for each 5g for Israel = 20 Rp., In the 2nd Weight Level = 40 Rp (1.1.1950-30.6.1952)