AI is getting better at this kind of thing.  Went to google, literally copied and pasted the last part of your question, 
got this - go to that link and you'll see the text below, plus reference links you can follow for a deeper dive:
For a Lee-Enfield No. 4 Mk I, the serial number and manufacturer's code are found on the left side of the receiver or the butt socket. The manufacturer is identified by a letter prefix and a specific serial number block, while the year is sometimes stamped separately. 
Locating the markings
- On the receiver: Look at the left-side wall of the receiver. On many No. 4s, the manufacturer's code and serial number are clearly stamped here.
- On the butt socket: Check the left-side of the butt socket (where the stock meets the action) for stamped markings.
- On other parts: The full serial number is often stamped on the bolt handle and, on all-matching rifles, may also appear on the magazine and fore-end. 
Deciphering the markings
British manufacturers
British No. 4 rifles were produced primarily by three factories, each with its own serial number system. 
- ROF (Royal Ordnance Factory) Maltby: Serial numbers typically start with the number 1 (e.g., 1xxxx), usually following a letter prefix.
- ROF (Royal Ordnance Factory) Fazakerley: Serial numbers typically start with the number 2 (e.g., 2xxxx), usually following a letter prefix.
- BSA (Birmingham Small Arms) Shirley: Serial numbers typically start with the number 3 (e.g., 3xxxx) or have a four-digit number with a single or double-letter prefix (1941–1943). 
North American manufacturers (Lend-Lease rifles)
During WWII, some No. 4 Mk I and Mk I* rifles were made under license in the US and Canada. 
- Savage-Stevens (USA): Rifles made under the Lend-Lease program have a "C" in the serial number (e.g., 0C1xxx) and may have a squared "S" proof mark. A "flaming bomb" mark also indicates US origin.
- Long Branch (Canada): These rifles have an "L" in the serial number (e.g., 0L1xxx). 
Date of manufacture
The date of manufacture is often stamped near the manufacturer's mark on the receiver or butt socket. The date will be a two-digit or four-digit year (e.g., 1943 or '43). 
How to use this information
To identify your rifle, you will need to:
- Find the serial number and manufacturer's mark on the rifle's receiver or butt socket.
- Match the number and prefix with the system used by the different manufacturers (e.g., a serial starting with 1 points to Maltby).
- Confirm the date by finding a year stamp near the other markings.