Bentley & Playfair.
In Douglas Tate's book 'Birmingham Gunmakers' he states that Bentley & Playfair may have produced sideplated boxlocks for Charles Hellis. Pg. 28, "Bentley and Playfair soon opened a shop at 60 Queen Victoria Street, London, and advertised that it had outlets in Paris, Capetown, and Melbourne. Their stock in trade appears to have been good quality boxlocks, often with sideplates identical to those seen on Windsor grade Charles Hellis guns. That fact has led at least one enthusiast to conclude that the Hellis guns - or at least the ones made in this particular grade - were built by Bentley & Playfair."
However, it's possible that these side plated boxlocks were made by S. Wright & Sons Ltd., Birmingham. Samuel Wright had originally started out working for Bentley & Playfair before starting his own company immediately after WW I circa 1918.
In Don Masters' book 'The House of Churchill', he states on pg. 281 that, "Wright's [S. Wright & Sons Ltd.] also made guns for Charles Hellis & Sons Ltd. After Charles William Hellis broke away and bought Geo. Hinton & Sons Ltd. of Tauton at the end of 1946, Wright's continued to make guns for both the companies of Charles Hellis and Sons Ltd. and Geo. Hinton & Sons Ltd. There was correspondence between Charles W. Hellis and Howard Wright concerning minor differences to the guns in order to distinguish Hinton guns from Hellis guns, although before World War II, the firm of Geo. Hinton & Sons was already a customer of S. Wright & Sons."
S. Wright & Sons Ltd.