Until the late 1980s, Morgan used Smith's Oil Pressure gauges. Then they switched to VDO. All were mechanical units until some time in the 1990s when they wisely switched to an electrical system.
The mechanical systems were accurate. They received the pressure right at the dash through a tube from the oil pump. Sadly, like the one-shot oiler, the systems are inherently risky as the tube is vulnerable to heat and other causes of breakage. When this happens, they cause pressurized oil to spray about the break point and empty the engine of motor oil (with dire consequences). When traveling, it is a pain to address this issue.
However, there are fittings that can "splice" the tube together with the appropriate pressure fitting. The systems come from the home oil furnace industry and are still used theough they have long since been outlawed for cars. So they can be had if you find a specialist in home oil furnaces. They are little brass fittings consisting of two pressure dowels (inside), a male threaded fitting and two fmeal threaded fittings. To repair, one must make clean cut ends, then take the fitting apart, slide the male fittings on each side of the tube, then place a dowel on a side, push a bit of the tube into the male threaded fitting and thread the femal bit on over the dowel an onto the male fitting. Now do the same on the other side and you have your splace. If there is insufficient length, the buy a bit of an extension from the same source your got the fitting and buy two fittings, one for each end of the splice. If securely attached to the car or motor, it makes a permanent repair...albeit with the same risks as the original.
These consist of a gauge which measures the resistance from sender (normally at the pump) which transforms varying pressure into varying resistance. This eliminates the tube filled with pressurized oil so that risks and consequence of that type of breakage is eliminated. Replacements gauge can be had, indistinguishable from the originals, that function electrically rather than mechanically. However, there are some issues that must be checked and confirmed. The sender, the oil pump and the gauge must match or be made to match so the readings make sense, the range of readings span the specific motor pressures, and the threads in the oil pump and sender fit (which can be done with adapters if necessary. These senders have all sorts of shapes..most often like a mushroom. Some will have two posts for wires (a ground/earth feed and a wire to the gauge) or one post (only a wire to the gauge as it assumbed the sender is grounded/earthed by the contact of the threads with the oil pump).
If the wire is broken or detached, the pressure will read very high or not at all depending on the car type. Simply repair it or run a new one.