India 1 Dollar / 2 Rupees 8 Annas Pattern Coin

[Oesho]

Above we can read quite a bit of guesswork why the patterns for the 2 ½ rupee coins were prepared. Basically, however, it was economic and capacity problem that lead to the proposal for introducing such a denomination. There was no intention for copying an existing (trade)dollar.
On page 86 Pridmore (Part 4, India, vol. 2 Uniform coinage) gives an explanation for these patterns:

1941 Patterns for a silver dollar
Two remarkable patterns were prepared in 1941 for a DOLLAR or 2 ½ rupee denomination. The word 'dollar' on the patterns had no relation to any decimal currency system of which the dollar coin is the unit, but followed the customary meaning applied to any coin approximating in value to the old Spanish or American coin. The sterling value of 2 ½ rupees on the Sterling Exchange standard in 1940 was 3/9d.
It is interesting to record that a suggestion to establish a dollar coin within the Indian monetary system had occurred at a much earlier date. Authority for the minting and issue of a DOUBLE RUPEE was contained in the East India Company's Act XVII of 1835, but had remained in operative and was eventually repealed by Act XIII of 1862. Shortly after this repeal the Government of Bombay, in September 1863, addressed the Secretary of State for India, suggesting the issue of a coin having the value of a double rupee calling it an INDIAN DOLLAR. (In 1863 the rupee sterling exchange was about 2/.) The grounds for the suggestion were based upon the importance of expediting the conversion of a large accumulated balance of bullion into coined money. Measured in terms of equal output in quantity, one minting press striking a coin of the value of two rupees would naturally produce double the value of a press striking a one rupee. The Government of India objected to the proposal, and on the 23rd August 1864 the Secretary of State for India upheld their objection.
In 1941, the basic reasons for the issue of a dollar coin were similar. Despite the enormous output of the Calcutta and Bombay Mints, pressure on metal stocks and machine capacity gradually built up, and consideration was given to measures that would not only lessen the demand for silver, but reduce the machine workload. It was in the light of these problems that the proposal was made to introduce a 2 ½-rupee coin which was provisionally called a dollar. The advantages were twofold:
(a)   a saving of metal would be effected, as each dollar would replace tow one-rupee coins and one half-rupee, which together had a combined weight of 450 troy grains, against the proposed weight of 225 grains for the dollar; and,
(b)   this would set free productive capacity for other purposes.
The project reached the stage where pattern cins were prepared for submission and approval by the Minister of Finance, with the intention of introducing an amendment to the Coinage Act to enable the dollar to be struck and issued and declared legal tender.
Before this could be done Japan entered the war and the proposal had to be abandoned.
Preparatory work on the patterns commenced in December 1940 or in the early part of 1941. They were designed by the Calcutta Mint Engraver, A.P. Spencer, and the reverses of both designs were cut by hand.