In using the name "Indemnity Act" to designate the law of March 3, 1863, contemporary usage has been followed. Senator Trumbull and others referred to the measure while under debate as the "Indemnity Bill," and the same designation appeared in the heading of the record, as well as in many other places. Cong. Globe, 37 Cong., 3 sess., pp. 1459, 1479.)
Source: James G. Randall, Constitutional Problems Under Lincoln, Revised Edition, 1951, Chapter 11, "The Indemnity Act of 1863," p. 189, footnote 8.
If you look at HR 591, as recorded on Dec 9, 1862, the working title was "An Act to indemnify the President and other Persons for suspending the privilege of the writ of Habeas Corpus and acts done in pursuance thereof." It also came with a preamble which explained the reasons for the proposed Act:
The Indemnity Bill was passed on March 3, 1863.
In the House:
March 2, 1863.THE CONGRESSIONAL GLOBE.
INDEMNITY BILL.
The SPEAKER. The hour of one o'clock having arrived the vote will now be taken, by previous order of the House, on agreeing to the report of the committee of conference on the indemnity bill
Mr. ALLEN, of Ohio, called for the yeas and nays.
The yeas and nays were ordered.
The question, was taken; and it was decided in the affirmative-yeas 99, nays 44; as follows:
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Source: Congressional Globe, 37th Congress, 3rd Session, March 2, 1863. (Page 1479)
In the Senate:
Journal of the Senate of the United States of America, 1789-1873
MONDAY, March 2, 1863.
Monday, March 2, 1863
Page 380
Mr. President: The House of Representatives has agreed to the report of the committee of conference on the disagreeing votes of the two houses on the bill (H. R. 591) to indemnify the President and other persons for suspending the privilege of the writ of habeas corpus, and acts done in pursuance thereof; and
Page 382
Mr. Trumbull, from the committee of conference on the disagreeing votes of the two houses on the bill (H. R. 591) to indemnify the President and other persons for suspending the privilege of the writ of habeas corpus, and acts done in pursuance thereof, submitted the following report:
The committee of conference on the disagreeing votes of the two houses on the bill (H. R. 591) to indemnify the President and other persons for suspending the privilege of the writ of habeas corpus, and acts done in pursuance thereof, and the Senate's amendment thereto, having met, after full and free conference have agreed to recommend, and do recommend, to their respective houses that the Senate recede from their said amendment, and agree to the said House bill, amended to read as follows, to wit:
AN ACT relating to habeas corpus, and regulating judicial proceedings in certain cases.
Page 397
The Senate resumed the consideration of the report of the committee of conference on the disagreeing votes of the two houses on the bill (H. R. 591) to indemnify the President and other persons for suspending the privilege of the writ of habeas corpus, and acts done in pursuance thereof; and
Page 399
A message from the House of Representatives, by Mr. Etheridge, its Clerk:
Mr. President: The House of Representatives has agreed to the report of the committee of conference on the disagreeing votes of the two houses on the bill (H. R. 591) to indemnify the President and other persons for suspending the privilege of the writ of habeas corpus, and acts done in pursuance thereof; and....
Journal of the Senate of the United States of America, 1789-1873
TUESDAY, March 3, 1863.
Page 400
Mr. President: The House of Representatives has passed the following bills, in which it requests the concurrence of the Senate:
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The Speaker of the House of Representatives having signed sixteen enrolled bills (S. 435, S. 483, S. 526, S. 534, S. 536, S. 537, S. 540, S. 543, S. 548, S. 557, S. 562, S. 564, S. 565, H. R. 226, H. R. 523, and H. R. 591) and two enrolled resolutions, (S. 129 and S. 133,) I am directed to bring them to the Senate for the signature of its President.
Mr. Howe reported from the committee that they had examined and found duly enrolled the following bills:
-- H. R. 226. An act to amend "An act to establish a court for the investigation of claims against the United States," approved February 24, 1855.
-- H. R. 591. An act relating to habeas corpus, and regulating judicial proceedings in certain cases.
-- H. R. 523. An act to promote the efficiency of the corps of engineers, of the ordnance department, and for other purposes.
The President pro tempore signed the enrolled bills (S. 435, S. 483, S. 526, S. 534, S. 536, S. 537, S. 540, S. 543, S. 548, S. 557, S. 562, S. 564, S. 565) and the enrolled joint resolutions (S. 129 and S. 133) yesterday reported to have been examined, and the enrolled bills (H. R. 226, H. R. 523, and H. R. 591) last reported to have been examined, and they were delivered to the committee to be presented to the President of the United States.