I think you have a fake cruciform. I have two Singer Bayonets.

#1 an ordinary Spike bayonet. NO Stampings on muzzle ring at all ( Would expect to see No4 MKII). Stamping on release button is N67= Singer. Paid £10 last year.

#2 is a 1942 Cruciform . Actual length of the cruiciform ( not blade length)= 71/2" or 19cms exactlly. Weight with out scabbard is 8OZ/ 220grms exactly.
The stamping in very small capitals on the locking button is SM/42 (SM over 42).Cost me £100 a few years ago.

Just for interest, the rarest Cruciform No4 MKI bayonet is the trials version made by RSAF at Enfield Lock in the 1930s. Sadly I don't know the stampings.Less than 20,000 were made. I was shown one by Terry Abrams the gunsmith, who has one with his experimental No4 rifle with a mag cut off!

Kind regards, Lester