I can't find any information online about any particular letters but there's a mention of the stamps coming out. It was around 2008 though wasn’t it? I'm not on the island so I haven't seen the exhibition. For research use here are some useful sites:-

Get on Ancestry this weekend while it's free:-

h ttp://home.ancestry.co.uk/

iMuseum has Manx newspapers subscription only though:-

h ttps://www.gov.im/MNH/services/

Free sites:-

h ttp://www.imuseum.im/
h ttp://www.iomfhs.im/lawsons/LawsonBMD/Main_Site_index.html
h ttp://www.isle-of-man.com/manxnotebook/
h ttps://familysearch.org/search
h ttp://manxbmd.com/