It's a Triumph (road-bike) ... with it's seat missing.! (1960's) Peut-être une 6T Thunderbird customisée (échappements relevés, guidon "corne de vache"...) Smaller, 350 or 500, it has the teardrop shaped casting in the primary cover. 3TA/5TA/early Tiger 100. Interesting mix of rigid frame suggesting late '40s? - with a tank with a 'mouth organ' badge (1957 or later?)
Triumph rear subframes are bolt-on, hardtail replacements have been available since the 60's.