Take a look at the Merry Devil title page again...
Note well the pictures. These are called printers devices, and were (surprise, surprise!) commonly used by London printers to help identify who had printed the material. Then, as now, a picture said a thousand words, but unlike now, the title page would commonly serve as a poster advertisement for the book (or so Dr. Menzer tells me). These days we put eye catching images on our posters to get people to read the text of whatever it is we're advertising, and I'm taking an imaginative leap that these devices might have served a similar purpose.
The great 20th century bibliographer Ronald McKerrow collected these device into his Printers and Publishers Devices in England & Scotland: 1485 - 1640, in which he describes and produces facsimiles of over 400 hundred devices. Neither of the devices on the Merry Devil title page is found within Printers and Publishers devices, but the lower of these two is close. That is indexed as device number 379 and has the following description: "(33 x 30 mm.) A mask with rings: the letters A. H. below. This is perhaps a cast ornament with the letters inserted." (144). In both cases it appears on material printed by A. Hart in Edinburgh. This is the device as McKerrow reproduces it:
A very striking similarity, eh? If it's not the same device, or at least produced from the same casting, that's one hell of a coincidence. If A. Hart was ever the printer for The Merry Devil, I would feel like this was less of a mystery.