The following analysis of the evolution of Boisselot pianos is
based on the instruments I have seen, and it is quite likely that
this text will evolve as new instruments come to light. The basic
models of Boisselot seem to follow the following chronological
evolution :

* The oldest grand pianos I know of (n° 502; 702) are short
(1.70  m) bi-cord instruments, without agraffes, similar in
conception to the « semi-grands » of Broadwood or Collard.
Although there is an English influence, it is obvious that Boisselot
studied the instruments that he sold before he made his own
pianos, and particularly those of Pape. The adjustable pilot and
the split mortise with adjusting screw are both Pape inventions,
that Boisselot would have seen on the square pianos he sold in
the late 1820’s.
These early pianos have three metal bars above the
soundboard, and a wood hitch plank covered by a brass plate.
The bridge is continuous.

*A second type of instrument (Hamamatsu; n° 833), slightly
longer (1.92 m), shows for the first time the case type
characteristic of Boisselot pianos : tulip legs and brass inlay all
over. Several  archaic features of the previous model disappear :
there is now a metal hitchpin plate, and  the bar covering the
dampers and serving as a stop no longer exists.

* A third type of instrument (n° 2394 for example), shows deep
structural modifications : it is tri-cord, with agrafes, slightly
longer, and the bars are now under the soundboard, probably to
avoid cutting into the bridge like on the earlier models, maybe
also for aesthetic reasons. This modification was unfortunately
not a very good one in retrospect, as a high proportion of these
pianos have structural problems : split tuning planks, and/or
tuning planks that have rotated and split the sides out.
The most common type of case is rosewood with brass inlay.

*A fourth type (n° 3460) is structurally identical, the only
difference is in the case type : the fallboard profile changes and
becomes more angular, and a lot of these instruments only have
one type of veneer, set off by intricate mouldings.

*At a later date, Boisselot come back to a more sensible
structural set-up, with bars above the soundboard.

This description of the evolution of Boisselot grand pianos is
obviously only valid for the basic models : they also made
concert models (2.45m), the earliest known is Liszt’s piano in
Lisbon, and continued to make bi-cord « pocket grands » (with
bars underneath) through to the 1840’s.

Detail evolution :

*Pedals : early Boisselot pianos have inverted pedals, which
makes things rather difficult for the pianist… I have no idea what
was the idea behind this, and Boisselot put the pedals back the
right way from c.1840.

*Dampers : N° 702 has a wooden bar above the dampers that
serves as a stop, but by n° 833 this has disappeared.
Original Boisselot damper heads are a very elegant round shape
(until 1844 at least), and the dampers themselves are made up of
layered strips of woollen cloth rather than felt (n° 2394 is still like
this).

*Music stands : This is a field where Boisselot was way above
any of his rivals. The triple music stand characteristic of
Boisselot grands, and so suggestive of playing chamber music,
appears in the first known models.
The fretwork of the panels is superb, with various patterns being
used in the earlier pianos. N° 833 is the first I know with the lyre
pattern that then became systematic.
Between n° 2394 and n° 2439, this pattern changed over to a
rosace reminiscent of that of a gothic cathedral….

*Lid shutters : N° 702 has lid shutters in the shape of garlands of
fruit and flowers.
N° 833 has horseshoe shaped shutters, decorated with a shell
and strap work. This type then alternated with a second type,
rose-shaped.

*Name plate : Boisselot first of all had orange labels, octagonal
in the 1830’s, rectangular in the early 1840’s.
In 1844 they used for a short period a gold plaque, that was then
replaced by very ornate tin and brass inlay, boasting the 1844
gold medal.