The divided back postcard dates from 1904 before then only the address could be written on the back.
|A fraction of one percent of portrait postcards were actually sent with a postage stamp through the mail.
If they were, the postmark date will hopefully be legible. Many more were sent enclosed in an envelope and the postmark shows only as a circular indent.
More often than you would imagine held at the right angle, in the right light, dates can still be read from these.
Stamps of the following value and design were applied to Inland Postcards as shown.
|1902-1904 King Edward VII - "blue green" halfpenny postage stamp
|1904-1910 King Edward VII - "yellow green" halfpenny postage stamp
|1911-1912 King George V - "Downey" ¾ Face halfpenny postage stamp
|1912-1918 King George V - "Profile" halfpenny postage stamp
|In 1918 a postcard stamp was doubled to one penny
|1918-1921 King George V - "red" penny postage stamp
|In 1921 the postage was increased to one penny halfpenny
|1921-1922 King George V - "brown" three-halfpenny postage stamp
|In 1922 it was reduced back to one penny
|1922-1934 The same King George V - "red" penny postage stamp
|1934-1936 King George V - "dark oval" penny postage stamp
|1936 (Abdicated) Edward VIII - "uncrowned" penny postage stamp
|1937-40 King George VI penny postage stamp
|In 1940 a postcard stamp was doubled to two pence
|1940-41 King George VI "dark orange" twopenny postage stamp
|1941-50 King George VI "pale orange" twopenny postage stamp
|1950-52 King George VI "pale brown" twopenny postage stamp
The increase in postage plus the introduction of purchase tax on postcards in 1940 ended the standard use of this format by most photographers.
Of course combinations of halfpenny and one penny stamps could be used for the higher postage rates.
The postcard is by far the most frequent format you will see for family studio portraits.