Rashi: This was after the plague that resulted from the sin of the Golden Calf. ¹ It can be compared to a flock of sheep of which the owner was particularly fond, which is stricken with pestilence. Following the plague, the owner asks the shepherd to count the remaining sheep to see how many are left.

Rashi: Because the latter took place on Rosh Chodesh Iyar, after the Mishkan was erected, whereas the former took place before they even began constructing the Mishkan. ¹

Rashi and Rashbam: It is referring to the sockets, ¹ which were manufactured from the first donation, ² which, in turn, formed the base of the Mishkan, in which the Avodah was performed.

Refer to 28:12:151:1.