The number is located on a sticker on the box and below the receivers.
The first 6 digits denotes the date and the following are the number of audits and products.

Example S/N 4B012017001254S

The first number is the last digit of production year and it is in this case 2004th year.
The second digit or letter represents the month, according to this table below, in this case the letter B which means November.

1 - January
2 - February
3 - March
4 - April
5 - May
6 - Jun.
7 - July
8 - August
9 - September
A - October
B - November
C - December

The third and fourth digits indicate the date of the month and in this case on 01
The fifth and sixth digit indicates the model at the next table in this case, figure 20 indicates that this is the DreamBox 500

09 - DM7020
01 - DM7000
10 - DM5600
11 - DM5620
04 - DM600
20 - DM500
12 -500 +
13 - DM800


Behind this label model going revision number and serial number 00.