The following images are from Leacy MG:
Fahrenheit BHA4586, various markets up to October 1968
Centigrade BHA4587, various markets up to October 1968
C-N-H BHA4900, used from October 1968 to August 76. Caerbont Automotive Instruments describes these Smiths gauges as '90-250F (30-120C)' whereas the earlier degrees C or F gauges show 90-230F and 30-110C.
BHA4900Z after-market from Moss Europe - note the subtle differences to the above, and not the cheapest by a long way.
V8 60psi BHA5227 used up to June 1974. Clausager says a 100psi gauge was used briefly after June 1974, possibly the 4-cylinder gauge, although it may have also had the V8 red and white cross-hatching of the H zone. This isn't listed in the Parts Catalogue. Caerbont show the temp range as being 30-120 C (90-250F) i.e. the same as for the 4-cylinder C-N-H gauges.
V8 80psi BHA5331 used after the 100psi gauge up to the end of production. Note the oil needle is angled up at rest and not horizontal like all the other gauges.
For completeness, the later electric single C---H gauge, used from August 77 on. Presumably the 'N' was dropped in a failed attempt to prevent owners getting paranoid about the range of and variation in 'normal' temperatures.
And the 77 and later oil pressure gauge.
The capillary sender - 'A' is the bulb that projects into the coolant under the thermostat. 'B' is the tapered seat on the flange that butts against the head. 'C' are the flat faces on the back of the sender flange and on the nut, the nut presses against the flange which presses the tapered side into the head to form a seal:
The temp gauge bulb projecting into the space below the thermostat. If the bulb won't pull out from the head the cover and stat can be removed to allow you to push it out using a bit more force. The orange looks awful but is just a thin film that wipes off, not sludge.
Carefully wind the excess capillary tube into three loops by the heater motor, and clip the tube using the lower heater valve bolt:
Make a graceful curve into the head: