Gents,
It is a modification, incidentally never approved by Douglas or P&W. It works well though
.
The fixed gill modifications are only approved in South Africa, Australia and New Zealand. I have done these conversions to fixed gills and from, ad nauseum. It's quite simple to unbolt the gill assembly and then manufacture 3 fixed sections, fixed at 11 degrees and then rivet them on. There is a cut out on the outboard side to fit around the bulge on the heater inlet cowling and either another cut out at the top to allow the top cowling to fit around the carb air scoops (only when using the ram/non-ram "saddle" airscoops), or a slight extension for the short carb airscoops, so cowl can fit snugly. Some fixed gill conversions have the heater cowling cut out on both sides, so that the cowlings can be interchangeable between left and right engines, and some have little holes in the gills, so that the accessory bay cowlings can be got at without too much stress.
The hydraulic lines are simply blanked with pressure blanks at the firewall and ditto for the selectors in the cockpit. Sometimes, we leave the selectors in the cockpit for show, some owners like this. In practice with the fixed gills in South Africa, we normally get a perfect cruise CHT of 200 deg. In cold conditions, this obviously drops, and in winter conditions cruise CHT can go as low as 160 deg, although this rare and the tendency is pretty much there with the normal gills too. As Old Smokey said, judicious use of "emergency rich" usually brings any high temp issues back into the green, but I've only ever experienced this once. This is bearing in mind that Take-Off CHT is permitted to go as high as 262 deg and cruise is permitted go as high as 232 deg.
It is true that the adjustments on the normal gills can be tedious and they can be a real pig to get aligned as the Maintenance Manual says, but once ok they work alright, just need looking at every check.