Old time sign board at a race track, what's "h ft"?

Not a 'sign board'.

As the drag drew up in the enclosure near the grand stand I glanced at the card to see the entries.

Cards listing the day's races were printed and made available to people at the racecourse.

Answered here:

Wessex Plate: The name of the race

50 sovs each: The cost to enter a horse (50 sovereigns)

h ft: Half forfeit. If a horse is withdrawn, then only 25 sovereigns are returned

with 1000 sovs added: The race sponsor has added 1000 sovereigns to the prize fund

for four and five year olds: The race is open to horses aged four and five years

Second, £300: The prize money is for coming second is £300, fixed (it is not a proportion of the prize fund)

Third, £200: The prize money for coming third is £200, fixed (it is not a proportion of the prize fund)

New course: The course it is run on (the venue clearly has more than one course)

(one mile and five furlongs): The distance run. There are eight furlongs in a mile, and a mile is 1760 yards, 5280 feet or approximately 1609 metres.

A 'sov' (sovereign) was a gold one-pound coin, also an alternative term for a pound (currency).

To be noted:

Although Conan Doyle was a sporting man, he knew little about horseracing, and mistakes in terminology and racing customs abound.

Discovering Sherlock Holmes (Stanford University)