I recently got the first horus heresy book from FW and I'm a bit confused about the boltguns some marines in the pictures are using.

There's multiple background pictures of marines in maximus armour holding the 'current' bolters (godwyn pattern?). The clearest expample is on page 126 where the marine in the foreground is holding a plastic bolter.

Is this correct from an in-universe point of view? Were these used in the heresy era?

I also noticed on their website that FW shows pictures of their iron warriors with these 'modern' bolters. But while that does not necesserarily imply that the pattern of the plastic bolter was used during the heresy (the pictures on their site don't depict a specific period in the 40k history), the horus heresy book does imply that you can use the modern bolters for that period (from an in-universe viewpoint).