For the timeframe 1969, 1971, 1972 and 1973, where I regularly used the ferry connections
- Dover <-> Ostende
- Harwich <-> Rotterdam or Hamburg
The regular procedure was:
- get off the train/ferry
- waiting in line in front of highten desk where an inspector sat
- present passport and alien registration book, when a foreigner resident
- pass by the desk to the customs area
It would have been difficult to get through without getting checked.
When leaving the residence permit was cancelled with a exit stamp, which was a triangle form.
On return a new residence permit was issued based on the previous one with an entry stamp in its present day form.
On the other side, with the exception of Hamburg, passports were stamped on entry.
The earliest UK enter/exit stamps that I have seen was 1928 on a US Passport.
Unfortunately the site is no longer active
It contained not only a day by day Journal of a 18 months tour around the world (UK, Ireland and Europe on a bicycle) , but also his 1928 Passport with all the visas.
For every country he travelled through a visa was needed and in the Journal he complained how expensive they were (he was on a very tight budget) .
Arriving from the US, through Liverpool he landed in Manchester.
As all other Empire stamps (Gibraltar, Port Said and India), the entry stamp was Oval.
The last of this type I have seen was 1958. The first present day, Rectangular, 1961.
The UK exit stamps were in this Passport and other 1930s Passports were of a triangle form which seemed to be in use until the exit controls ended.
From what I have read these controls started during WW1.