Here’s the gen for converting a standard Dapol early headcode Class 22 to a late batch loco. Principles would be the same... with suitable adjustments ... for the retro fitted headcode box locos.
First - before doing anything else - get what you need - including pictures of your chosen loco.
Class 40 split box etches from Extreme Etchings - EEDP40-18. Also wipers and flush glazing
Class 21/22/29 nose end detail pack .... A1 models on Ebay
0.31mm ns wire
Silver Tay lamp irons on EBay
Brass tube, sheet and plasticard sheet and strip.
Whilst you are waiting for those remove existing hand rails and lamp irons. It’s the headcode boxes that are the main work
Pilot drill a hole through the moulded circular top vents either side of the doors just below the driver/second man front windows. These together with the head/tail lamp holes will serve to give you location reference points later.
Then carefully carve away the head code box surrounds flush with the surface - and all other raised details on the nose end. Leave the outline of the headcode boxes visible. Take a small flat file - handily the most common size is exactly the width of the headcode box aperture - and extend the aperture down 1mm to the lower limits of the existing box you have just removed - the outline will still be visible.
Once you are happy with this then clean off all moulded nose end detail and fill existing hand rail and lamp iron holes with squadron putty. Rub down to a smooth surface - the nose doors are recessed, leave them visible as a template for the new doors you are going to make.
First job is to sort the headcode boxes. The etches are thin so need to be soldered on to brass sheet....0.5mm is about right. Drill centre of aperture and file out to open up box to internal edge of etching box. The frames are too heavy so file down width from outside too ensuring final results are square and consistent. Sweat on a length of 0.31mm ns wire along the top edge of the box to represent the hinge. Then mount boxes in place on model aligning Internal lower edge of box with internal edge of aperture. Superglue good for all brass and NS fittings but use gel not thin stuff. Ensure boxes are square and height is right relative to reference points and lateral orientation is correct relative to doors and edge of nose.
Next job are nose end doors. Thinnest plasticard cut and filed to shape and attached with plastic weld. I then prefer to prime before adding any more fine detail as any defects can be corrected before more detail is added.
Now for fine details. Use the reference point holes as drilled above to get the positions correct. Top air vents are brass tube drilled out to get thinner walls. Tube used to get protruding effect as per prototype. Duct in nose doors is a small brass washer recessed in to door, and tail lamp casings are brass signal cranks drilled and filed to shape. The only item used from the A1 models kit in the end were the horn grilles. Headcode box latches and small angled pad above doors are plasticard.
Handrails are 0.31 NS wire, as are door and headcode box hinges. Had to fabricate lamp irons by soldering up two irons off the Silver Tay thread...they are fine, made them handed although the prototype ones have central uprights. Could not get those robust!
Off the nose ends, two things to check.... drivers name card holder , no longer present on this loco....and any partial blanking of roof boiler port, use thin plasticard. Buffers .... Heljan Hymek are an improvement, fit them and buffer beam detail if required.
Assuming you are happy with all of that, it’s out with the paint. Jim used air brush to apply Precision primer, gloss rail blue and warning panel yellow. Then Fox transfers, and works plates if present. Precision Labels will custom print correct data panels. Check photos for missing valances, spray them and fit. Then Matt varnish and weathering to taste.
Final touches .... Precision Labels headcodes, then Shawplan flush glazing all over... and wipers from same source. Cab interior modified as lighting no longer lines up, crew are Bachmann 1950s loco crew, they look the part.
Just got sound chip and coupling loops to fit before she is good to go.