7.5mm toshiba
-x  336.00 -y  274.69 -a 9.826702635e-01 -b 2.333436093e-07 -c -1.157903449e-13

7.5mm toshiba; filt2d -g5.1
-x  167.56 -y  137.62 -a 9.825952577e-01 -b 9.390458504e-07 -c -1.930069583e-12

seems like x/2, y/2, a, 4*b, 16*c
-x  168.0 -y  137.345 -a 9.826702635e-01 -b 9.3337e-07 -c -1.8526e-12

single field captured on SGI (edgware) after upsampling:
-x  312.06 -y  228.25 -a 9.760263567e-01 -b 2.779294222e-07 -c 2.969901299e-13

elmo MN401 with 7.5mm lens, captured on Ajeco and transmitted to WWW page:
-x  300.81 -y  246.31 -a 9.762688152e-01 -b 2.648105984e-07 -c 3.396371983e-13

netcam8.5_E/deletemedots
-x  321.75 -y  213.00 -a 9.583439350e-01 -b 6.730919385e-07 -c -1.2 29196424e-12

identity: test:
swarp -w -x 320 -y 240 -a 1 -b 1e-40 -c 1e-40 -o outfile infile
??? but Segmentation fault