After Patrick Stirling died in office (11th November 1895) his Chief Clerk & Locomotive Accountant, John Robert Whitworth Matthewman (1841-1900), became acting Locomotive Engineer though no formal appointment by the Board has been found.  In fact almost all we know of him is through the 10-yearly census returns.  Construction work already on order in particular the 1021 Class 0-6-0s with their larger boilers proceeded as planned.  At the GNR Board meeting held on 6th December 1895 it was announced that Henry Alfred Ivatt intended to take over as Locomotive Engineer on 1st March 1896 and that the vacant post of Works Manager & Chief Assistant to the Locomotive Engineer was to be filled by Douglas Earle Marsh from Swindon. 

The date of Marsh's arrival at Doncaster has not so far been traced.  In the meantime the four following boiler design features were developed under Matthewman, with related boiler drawing numbers in the N series quoted:

Regulator valve placed in a dome on top of the boiler instead of inside the smokebox (see N69).
Foundation rings (also known as “water-legs”) which separated the outer and inner fireboxes at the bottom reduced in width from 3in to 1¾in to increase the grate area (see N71).
Working pressure increased from 160 to 170 lb/sq in (see N71).
Firebox throat-plate of the No. 1 Class 4-2-2s inclined forwards to lengthen the grate (see N72).

The sequence of development was as follows.

N65 (15th January 1895) 4ft-5in. diam. domeless, 196 x 1¾in. diam. tubes, for new mineral 0-6-0s (allocated Nos. 1021-26, amended later to 1021-30, built 2-3/1896).  General arrangement drawing P36 (12th January 1895) applied.  Only ten boilers were constructed and fitted to mineral engines Nos. 1021-30 (2-4/1896).  Superseded by N71.

N66 (November 1895) 4ft-2½in. diam. domeless, 174 x 1¾in. diam. tubes, for new standard 0-6-0s (Nos. 1031-45 allocated later (built by Dubs 6-7/1896).  General Arrangement drawing P37 of November 1895 applied.

N67 (February 1896) arrangement of tubes at firebox end for “tank engines” sic Nos. 1031-45.

N68 (February 1896) similar to N67 but at smokebox end.  However Nos. 1031-45 were tender engines and 1046-60 were tank engines, so there was an error somewhere.  It seems most likely that “tank engines” in the register should have read simply “engines”.

N69 (February 1896) arrangement of 180 x 1¾in.tubes at firebox end for 8ft. engines with domed boilers.

N70 (February 1896) similar to N69 but at smokebox end.  See N72.

N71 (18 February1896) 4ft.-5in. diam. domeless, 215 x 1¾in. diam. tubes, narrow foundation rings, working pressure raised to 170 lb./sq. in, for “standard” 5ft.-0in. wheel goods engines.  Only 20 boilers constructed, fitted to 1081 Series 0-6-0 (built 9-12/1896) plus 10 spares.  Superseded by N77 (with domes).

N72 (February 1896) domed boiler, enlarged firebox, narrow foundation rings, for 8ft 4-2-2s (incorporating N69 and N70 for tubes).  The first boiler was fitted to No. 93 (ex-works 18th June 1896).

This sequence of events points to Matthewman having initiated the domed boiler on the GNR  But the range of changes suggests it is possible that the Board had presented Matthewman with a copy of Ivatt’s proposals (provided by him at his interview), for Matthewman to discuss with the Chief Draughtsman, which in any case is what Ivatt would have done.  The introduction of narrow foundation rings was a mistake as sludge accumulated in the narrow space and Ivatt back-tracked on this feature (December 1901). 

Tentative construction was probably set out during the interregnum with engine numbers allocated as follows.

1031-45 Stirling 0-6-0 with Stirling 4ft-2½in diam. boiler, these appeared from June 1896, built by Dubs
1046-60 Stirling 0-6-0ST with 4ft-0½in diam. boiler, these appeared from July 1896, built by Neilson
1061-70 2-4-0 with 4ft-5in diam. boiler, these appeared from February 1897
1071-80 2-4-0 with 4ft-5in diam. boiler, these appeared as 4-4-0s from May 1897
1081-90 0-6-0 with 4ft-5in diam. boiler, these appeared from September 1896
1091-1100 0-6-0 with 4ft-5in diam. boiler, these appeared from December 1897, built by Dubs

For a start material for twenty 4ft-5in domeless boilers (to N71) was ordered, the intention no doubt being to fit these to the 2-4-0s. 

After Ivatt's arrival the following boiler drawings were made, and this meant a change to the construction plan.

N73 (March 1896) riveted joints for N72 boiler.
N75 (March 1896) 4ft.-5in. domed boiler for 5ft.-6in. 0-4-2s, not used.  Replaced by N77.
N76 (March 1896) 4ft.-5in. domed boiler for 6ft.-6in. 2-4-0s, not used.  Replaced by N77.
N77 (July 1896) 4ft.-5in. domed boiler 215 x 1¾in. diam. tubes, for goods and passenger engines.
N78 (August 1896) bottom corners of firebox etc for N77 boiler.
N79 (1 October 1896) dome cover for N77 boiler.

The construction plan for the Doncaster-built engines was amended.  The 0-6-0s Nos. 1081-90 were given preference and they took ten of the N71 domeless boilers on order.  Completion of the remaining N71 domeless boilers on order was then "put on hold" so that for the time being the N77 domed version took priority.  The remainder of the plan followed the intended order, but the second batch of 2-4-0s appeared as 4-4-0s (Nos. 1071-80).  Eventually the last ten N71 domeless boilers were completed and fitted in 1897 (one engine), 1898 (five engines) and 1899 (four engines). 
As a reminder of the interregnum period some of these domeless boilers had a long life, with four surviving at Grouping on 0-6-0STs (Nos. 494A until August 1929, 633 until April 1926, 673 until May 1927, 912 until December 1930).