Printed image is not center-aligned for smaller paper-sizes on Laserjet 1100

LaserJet 1100 has a paper guide that guides the paper to the center of the printing engine horizontally. The hpcups.drv PPD compiler input file does not seem to respect this speciality of the LaserJet 1100 which results in left-aligned image (wrt. the printable area of the engine) and thus part (or whole) of the image is printed off-paper (since the small paper is center-aligned wrt. the engine).
The used driver:
ii printer-driver-hpcups 3.18.12+dfsg0-2 amd64 HP Linux Printing and Imaging - CUPS Raster driver (hpcups)
Also checked in the upstream/latest branch on salsa.debian.org: https://salsa.debian.org/printing-team/hplip/-/blob/ab0ec267802e69a46a52e78c85d8f90dc241f7d0/prnt/drv/hpcups.drv.in

I am fixing this issue via creating a LaserJet 1100-specific entry in hpcups.drv (checking the toolchain that generates this file) and using the cupsInteger1 attribute for the setpagedevice command in paper size specifications (or elsewhere if can be done genericly). Since the code in prnt/hpcups/Encapsulator.cpp: Encapsulator::sendJobHeader() doing centering the printed image (if the printable area size is specified in cupsInteger1) also has a bug I am fixing that too via moving the cursor positioning PCL command (*p#X) before the Start Raster Graphics command (*r1A) since it has no effect if issued after the Start Raster Graphics.
I am willing to contribute the fix to upstream.
If there is a description about how to contribute (where is the upstream VCS, what is the process to be followed, etc.), please send me a link.