Unprecedented?
I think you yourself wrote that it was 3 weeks after Clinton was impeached before the Senate had the Managers come.
If not you - someone else.
It was. There is nothing in the Congressional record of the day the impeachment resolution passed to suggest the House went to the senate that day.
But it did. The record appears in the first day of the next Congress.
I learned that about an hour ago. Checked the Congressional record myself, found a couple entries that the House went to the senate secretary (the senate was had days earlier adjourned sine die) and provided the notification.
Messages from the House received subsequent to sine die adjournment - Jan 6, 1999
Under the authority of the order of the Senate of January 7, 1997, the Secretary of the Senate, on December 19, 1998, subsequent to the sine die adjournment of the Senate, received a message from the House of Representatives announcing that the House of Representatives has impeached for high crimes and misdemeanors William Jefferson Clinton, President of the United States; the House of Representatives adopted articles of impeachment against William Jefferson Clinton, which the managers on the part of the House of Representatives have been directed to carry to the Senate; and Mr. Hyde of Illinois, Mr. Sensenbrenner of Wisconsin, Mr. McCollum of Florida, Mr. Gekas of Pennsylvania, Mr. Canady of Florida, Mr. Buyer of Indiana, Mr. Bryant of Tennessee, Mr. Chabot of Ohio, Mr. Barr of Georgia, Mr. Hutchinson of Arkansas, Mr. Cannon of Utah, Mr. Rogan of California, and Mr. Graham of South Carolina, have been appointed as managers.