A No5... but what kind?

No one?

The rifle has been scrubbed. Fazackerly would never had stamped just a serial number into the reciever like that. Your main concern is the asterisk looking stamp on the top of the reciever.  That indicates that rust was found in the bore somewhere.

The AK stamp on the back of the reciever has something to do with the Greek army IIRC. The BNP is part of the sold out of service into the civilian market stamp. Unless that has been scrubbed as well, you should also find on the barrel somewhere,  .303  2.222"  18.5 Tons Per □" as well as ENGLAND stamped somewhere on the reciever. One other item of contention you need to address is proper headspace, making sure that it does not close on a .074" headspace gauge.  The only reason I say this is because of your bolthead has a #3 stamped on it. That is as long as those boltheads get and are only used as a stopgap to keep the rifle in the fight so to speak until it comes back in for a barrel replacement. With that being said and with what I had stated about possible rust in the bore earlier, you definitely need that rifle checked over by a reputable gunsmith...

There really is no rhyme or reason as to why a rifle gets scrubbed of the original serial numbers and everything else for that matter,  especially if it ended up at the Indian Plant. The rifle would have been sold out of service regardless of the rust found in the bore as denoted by the * or the #3 bolthead. The screen grab of your bolthead has the 3 circled in red. The boltheads can be found with four different numbers. 0, 1, 2, and 3 with each one typically  being .003" longer than the previous number with 3 being the longest, typically. When a rifles headspace gets to the point it is fitted with a #3 bolthead, you should have it checked using a .074" headspace gauge. If the bolt closes on that gauge it would best be advised to start looking for a replacement barrel. As far information goes regarding the No5Mk1,  the Lee Enfield book by Ian Skennerton would be your best bet...